Writing a thesis in English as a second language is a remarkable achievement—but it can also feel overwhelming. You’ve done the hard work: researching your topic, analysing your data, and developing your ideas. But when it comes to writing it all down, even strong research can be undermined by simple grammar mistakes. For ESL (English as a Second Language) students, these errors are not a reflection of ability—they’re a natural part of working in another language. That said, in academic writing, grammar really does matter.
A well-written thesis is not just about what you say—it’s about how clearly and professionally you say it. Grammar errors can confuse your reader, weaken your argument, or distract from the originality of your research. Even small mistakes—like using the wrong preposition or article—can affect how your work is perceived, especially by examiners who are evaluating clarity and fluency as part of academic standards.
The good news? These errors are fixable—and you’re not alone. Most ESL writers struggle with the same types of grammar issues, and with the right guidance, they’re easy to spot and correct. In this article, we’ll walk you through the ten most common grammar mistakes ESL students make when writing a thesis. We’ll explain what they are, show you how to fix them, and share tips to help you write with more confidence.
Want even more support? Check out our Thesis Proofreading Services designed specifically for non-native English speakers.
See also: Writing an Academic Thesis in a Second Language
Top Ten ESL Grammar Errors
1. Subject-Verb Agreement
One of the most common grammar problems ESL thesis writers face is subject-verb agreement. In English, the subject and verb must agree in number—singular subjects take singular verbs, and plural subjects take plural verbs. This might sound simple, but it becomes more complex in long or technical sentences, especially when modifiers or unfamiliar noun forms get in the way.
For example:
❌ The results of the experiment was significant.
✅ The results of the experiment were significant.
In the incorrect sentence, “was” appears to match the word “experiment,” but the true subject is “results,” which is plural—so the verb should be “were.”
Another tricky area involves collective nouns or phrases with embedded clauses. Consider the following:
❌ The committee have decided to postpone the review.
✅ The committee has decided to postpone the review.
Even though “committee” refers to a group of people, it is treated as a singular noun in American English, so the verb must be singular (“has”). In British English, treating collective nouns as plural is more common—so consistency matters depending on the academic style you’re following.
Subject-verb disagreement often goes unnoticed in early drafts because the sentence might “feel” correct when you read it quickly. That’s why it’s essential to double-check every long or complex sentence, especially those that include relative clauses, parenthetical statements, or unusual word order.
To make proofreading easier, try this tip: identify the main subject and then match it directly with the verb—ignoring everything else in the sentence for a moment. This simple technique can help you spot agreement errors before they weaken your writing.
2. Article Usage (a, an, the)
Articles are a constant source of confusion for ESL writers—especially for those whose native language doesn’t use them. In English, we use a, an, and the to signal whether a noun is general, specific, countable, or uncountable. Knowing which article to use (or whether to use one at all) can be surprisingly difficult.
Let’s break it down with some examples:
❌ She conducted an extensive research into the topic.
✅ She conducted extensive research into the topic.
In this example, “research” is an uncountable noun, so it doesn’t require an article. Many ESL writers mistakenly treat it as countable because it seems measurable—but grammatically, it’s not.
Here’s another common mistake:
❌ The participants completed a survey about the course experience.
✅ The participants completed the survey about the course experience.
“A survey” suggests any random survey, but in a thesis, it’s usually a specific instrument you designed. Therefore, “the survey” is more appropriate.
General rules to remember:
- Use a/an for something general or mentioned for the first time: “a theory,” “an idea.”
- Use the for something specific or already mentioned: “the theory outlined in Chapter 2.”
- Do not use articles with most uncountable nouns when speaking generally: “evidence,” “literature,” “information,” “research.”
Another example:
❌ This study will provide the evidence for the claim.
✅ This study will provide evidence for the claim.
If you’re referring to evidence in general, you don’t need “the.” But if you’ve previously specified which evidence you’re talking about, then “the evidence” is appropriate.
Articles are subtle but powerful. Misusing them won’t always cause confusion, but frequent errors can make your writing seem unpolished or non-native. Take the time to review article rules, and when in doubt, consult trusted academic sources or dictionaries like the Cambridge Dictionary to check whether a noun is countable or uncountable.
3. Tense Consistency
Switching between tenses is one of the easiest grammar mistakes to make—especially when writing over several weeks or months. Many ESL thesis writers start a paragraph in the past tense and unintentionally shift to present or future tense midway through. These tense shifts can confuse the reader and weaken the flow of your argument.
Let’s look at an example:
❌ This chapter discussed the findings and shows that the results are significant.
✅ This chapter discusses the findings and shows that the results are significant.
✅ This chapter discussed the findings and showed that the results were significant.
Both corrected versions are technically fine—the key is that the tenses match. In the first, present tense is used for a general academic tone; in the second, past tense is used to describe work that was already completed. Whichever you choose, the important thing is to be consistent.
In academic writing, here’s a general guide:
- Use past tense to describe methods and results: “Data were collected…”
- Use present tense for general truths and interpretations: “The findings suggest…”
- Use present perfect to link past events to the present: “Researchers have debated…”
Here’s another example of inconsistent tense:
❌ Participants were selected randomly and the data is analysed using SPSS.
✅ Participants were selected randomly and the data was analysed using SPSS.
When writing your thesis, pick a primary tense for each section (usually past tense for methods/results, present tense for interpretation/discussion) and stick with it. Rereading your work aloud can help you spot sudden changes in tense that you may have overlooked during writing.
Tense consistency may seem small, but it has a big impact. It helps readers follow your logic without getting distracted by sudden shifts in time.
Tip: Create a “tense map” for your thesis. Note which tense you’ll use in each chapter or section, and refer to it when editing.
See more: What Tense Should I Write my Thesis in?
4. Preposition Problems
Prepositions may be short, but they can cause big problems—especially for ESL writers. These tiny words (like in, on, at, of, about, to, for) carry a lot of grammatical weight and don’t always follow logical rules. Often, the correct preposition depends not on meaning alone but on usage and convention, which can be frustrating for those learning English.
Let’s look at some examples of common mistakes:
❌ The paper focuses about climate policy.
✅ The paper focuses on climate policy.
❌ The participants responded on the survey.
✅ The participants responded to the survey.
These may seem like small errors, but incorrect prepositions can make your writing sound awkward or unclear to native readers. In academic writing—especially at the thesis level—precision matters.
Here’s another example:
❌ This study examines the effect in social media use.
✅ This study examines the effect of social media use.
Many prepositions are collocational, meaning they’re linked to specific words. You don’t just apply something, you apply for a grant, apply to a program, or apply knowledge in a field. Each verb or noun may require its own preposition, and sometimes there’s no clear “rule”—you simply have to learn the correct pairings.
To help build your confidence with prepositions:
- Read published journal articles in your field to see how phrases are used
- Keep a list of common academic collocations
- Use the Oxford Collocations Dictionary or similar resources to check combinations
And finally, trust your proofreading process. During revision, highlight prepositions and ask: Does this word belong here? If it feels off, double-check it.
Related reading: Using Plain English in Academic Writing
5. Sentence Fragments
A sentence fragment is a group of words that looks like a sentence but isn’t one. It’s missing a subject, a verb, or a complete thought. In academic writing, fragments can interrupt the flow, confuse your reader, and make your thesis appear unfinished or rushed.
For ESL writers, fragments often appear when trying to write more “academic-sounding” sentences or when attempting to use complex structures. But sometimes, in the effort to sound formal or concise, a key part of the sentence gets left out.
Here’s a common example:
❌ Although the study identified significant patterns.
✅ Although the study identified significant patterns, further analysis is needed.
The first version sets up a dependent clause (“although…”) but never finishes the idea. The reader is left waiting for the rest of the sentence.
Another example:
❌ Such as the methods used in previous experiments.
✅ This study replicates previous findings, such as the methods used in earlier experiments.
Fragments like this often occur when listing examples or giving explanations. Writers introduce an idea but forget to attach it to a main clause.
How to fix sentence fragments:
- Read your sentence aloud—does it sound complete?
- Look for dependent words like although, because, since, such as, which—if they’re present, make sure the sentence continues with a full independent clause.
- Check that each sentence has a subject and a verb and expresses a complete thought.
Even native English speakers make this mistake—especially in early drafts. But in a thesis, fragments can damage the clarity and authority of your writing. Always revise for sentence completeness during the final stages of editing.
Tip: When in doubt, combine shorter fragments with neighbouring sentences to ensure logical flow.
6. Run-On Sentences and Comma Splices
Run-on sentences and comma splices are two common grammar issues that often sneak into the writing of ESL thesis writers—especially when trying to connect ideas smoothly. While the intention is usually good (to sound fluent or academic), the result can be confusing and grammatically incorrect.
A run-on sentence occurs when two or more independent clauses (complete sentences) are joined without proper punctuation or a connecting word:
❌ The participants were interviewed the results were then analysed.
✅ The participants were interviewed, and the results were then analysed.
A comma splice is a similar error. It happens when two independent clauses are connected only by a comma:
❌ The theory is widely accepted, it is rarely applied in practice.
✅ The theory is widely accepted. However, it is rarely applied in practice.
✅ The theory is widely accepted, but it is rarely applied in practice.
Both errors can make your writing hard to follow or appear unpolished. Fortunately, they’re easy to fix once you know what to look for. You can correct a run-on sentence or comma splice by:
- Splitting the sentence into two with a full stop
- Using a semicolon if the ideas are closely related
- Adding a coordinating conjunction (e.g., and, but, so, or)
- Restructuring the sentence into one complete thought
Here’s another example:
❌ This model has been used in multiple studies, the results vary significantly.
✅ This model has been used in multiple studies, but the results vary significantly.
✅ This model has been used in multiple studies. The results vary significantly.
Tip: When proofreading, look for long sentences with two distinct ideas. Ask yourself: “Could this be split into separate sentences?” If yes, you might be dealing with a run-on or comma splice.
Related: How to Proofread a Thesis Like a Pro
7. Word Form Errors (e.g., affect vs. effect)
Word form errors occur when the wrong grammatical form of a word is used—often by confusing a noun with a verb, an adjective with an adverb, or two words that sound similar but function differently. These mistakes are particularly common among ESL writers because English vocabulary includes many lookalike words with entirely different uses.
One of the most well-known examples is:
❌ The new policy had a positive affect on student motivation.
✅ The new policy had a positive effect on student motivation.
In this case, “effect” is the correct noun—the result of something. “Affect” is typically used as a verb (to influence), not a noun.
Another common mistake:
❌ The researchers aimed to analysis the results.
✅ The researchers aimed to analyse the results.
Here, “analysis” is a noun, but the sentence needs a verb—“analyse.”
Let’s look at a few more examples:
❌ This method is very use in practical settings.
✅ This method is very useful in practical settings.
❌ The team conducted a detailed analyse of the data.
✅ The team conducted a detailed analysis of the data.
These errors can confuse readers and interrupt the flow of your argument. They also tend to make your writing feel less fluent or academic—even when your ideas are strong.
To avoid word form errors:
- Learn the most common academic word families (e.g., analyse / analysis / analytical / analytically)
- Use tools like Grammarly or a good academic dictionary to check usage
- During proofreading, highlight key academic terms and double-check you’re using the correct form for the sentence
8. Misplaced Modifiers
A modifier is a word or phrase that provides more information about another word in a sentence. A misplaced modifier happens when that information is in the wrong place—making the sentence confusing or, sometimes, unintentionally funny.
These errors are especially common in academic writing by ESL students because English word order is strict and not always intuitive. When a modifier is in the wrong spot, it can make it unclear what is being described.
Let’s take a look at an example:
❌ The student submitted the assignment to the professor that was incomplete.
✅ The student submitted the incomplete assignment to the professor.
✅ The student submitted the assignment that was incomplete to the professor.
In the incorrect version, it sounds like the professor was incomplete, not the assignment! The confusion comes from the placement of “that was incomplete.”
Another example:
❌ Nearly the professor received 100 emails per day.
✅ The professor received nearly 100 emails per day.
Here, the word “nearly” is modifying “the professor” instead of “100 emails.” While the sentence is short, the meaning changes entirely based on word order.
How to avoid misplaced modifiers:
- Place modifiers as close as possible to the word they describe
- Reread your sentence and ask, “What is this phrase describing?”
- If it’s unclear or accidentally humorous, it likely needs to be repositioned
Correct modifier placement makes your writing easier to follow and helps ensure your reader interprets your meaning as intended—especially in complex academic arguments.
Tip: Reading aloud slowly often helps you hear misplaced modifiers before you see them.
9. Parallel Structure
Parallel structure—also known as parallelism—is the grammatical practice of keeping similar sentence elements in the same format. This is especially important when you’re writing lists, comparing ideas, or outlining multiple actions. Inconsistent structure can make sentences sound awkward, unbalanced, or unclear—something that often goes unnoticed by ESL writers.
Here’s an example of a sentence with poor parallel structure:
❌ The goals of this thesis are to explore cultural influences, analysing social dynamics, and how media shapes identity.
✅ The goals of this thesis are to explore cultural influences, analyse social dynamics, and examine how media shapes identity.
In the corrected version, each item in the list begins with the to + verb structure: to explore, to analyse, to examine. This makes the sentence easier to read and more professional.
Here’s another common example:
❌ The researcher was responsible for collecting data, reviewing the literature, and the analysis of results.
✅ The researcher was responsible for collecting data, reviewing the literature, and analysing the results.
Mixing verb forms (e.g., “collecting,” “reviewing,” and “the analysis”) disrupts the rhythm and weakens the clarity of your writing.
Parallel structure is especially important in:
- Lists or bullet points
- Sentences using coordinating conjunctions (and, or, but)
- Thesis statements and research aims. Check out our free thesis statement generator for help with this.
- Headings and subheadings
To fix parallel structure:
- Identify the pattern (e.g., all verbs, all nouns) and make sure each element matches
- Avoid mixing gerunds (-ing forms) with infinitives or noun phrases in the same list
- Read each item separately to check if it sounds grammatically complete
Maintaining parallelism not only improves grammar—it also enhances clarity, logic, and flow. In academic writing, this can make a big difference to how your argument is received.
Related: Structuring Your Thesis
10. Incorrect Use of Relative Clauses
Relative clauses—those beginning with who, which, that, whose, where, and when—are used to add extra information about a noun. While they’re useful for building more detailed, academic-sounding sentences, they can cause problems if they’re misused, mispunctuated, or incorrectly structured.
Here’s a common mistake:
❌ The survey, which was conducted among 200 participants revealed key trends.
✅ The survey, which was conducted among 200 participants, revealed key trends.
In the incorrect version, the relative clause is missing commas, which creates a confusing sentence. Relative clauses are divided into two types: defining (essential to the meaning of the sentence) and non-defining (extra information).
-
Defining clause (no commas):
✅ The participants who completed the survey were included in the analysis. -
Non-defining clause (use commas):
✅ The survey, which was conducted online, received 200 responses.
Another common ESL error involves choosing the wrong relative pronoun:
❌ The article who explains the theory is outdated.
✅ The article that explains the theory is outdated.
“Who” is used for people, while “that” or “which” should be used for things. These small distinctions are easy to miss—but they’re important for maintaining accuracy and fluency in your writing.
To master relative clauses:
- Learn when to use commas (non-defining clauses) and when not to (defining clauses)
- Use “who” for people, “which” for non-essential clauses about things, and “that” for essential clauses
- Make sure the relative clause is directly next to the noun it modifies to avoid ambiguity
Bonus tip: If your sentence feels overly long or confusing with a relative clause, break it into two. Clarity is more important than complexity.
Final Tips and Encouragement
If English isn’t your first language, writing a full academic thesis can feel like climbing a mountain—but grammar doesn’t have to be your stumbling block. The issues we’ve covered—subject-verb agreement, articles, tense consistency, prepositions, fragments, run-ons, word forms, modifiers, parallel structure, and relative clauses—are some of the most common traps for ESL writers. The good news is that they’re all fixable, and every mistake is an opportunity to improve your skills.
Remember, strong writing doesn’t mean perfect grammar from the first draft. It means being open to revising, learning from feedback, and improving over time. Read your thesis aloud. Use helpful tools like Grammarly, but don’t rely on them completely. Ask others to review your work if possible, and don’t hesitate to seek professional help when you need it.
At Vappingo, we specialise in Thesis Proofreading for Non-Native English Speakers. Our editors understand the unique challenges ESL writers face, and we’re here to help you express your ideas clearly, confidently, and correctly—without rewriting your work or losing your voice.
You’ve done the research. You’ve written the chapters. Now it’s time to present your ideas with clarity and polish. With a careful eye and a little support, your thesis can be a true reflection of your hard work and academic potential.