Common Grammar Mistakes in Thesis Writing (And How You Can Avoid Them)

Introduction

Ever found yourself re-reading a sentence in your thesis ten times because it just didn’t sound right—but you couldn’t figure out why? You’re not alone.

Thesis writing is hard enough without grammar tripping you up at every turn. A 2018 study from Grammarly showed that students make an average of 15–20 grammar errors per 1,000 words in academic writing. That’s a lot of room for improvement—and for clarity!

You’ve probably put countless hours into your research. But if your grammar gets in the way, your brilliant ideas might never shine through. The good news? You can fix this. Most of the mistakes grad students make in academic writing aren’t even about intelligence—they’re about habit, oversight, or simply not having someone point it out.

In this guide, you’ll learn about the most common grammar mistakes in thesis writing—and, more importantly, how to avoid them without second-guessing every sentence you write.

Misusing Tenses

You tend to flip between past, present, and future tenses without realizing it.
Research results should often stay in the past tense (e.g., “The study showed…”).
Theories and general facts usually stay in the present tense (“Einstein’s theory explains…”).
Mixing tenses confuses your reader and weakens your argument.

Let’s say you’re describing your methodology and suddenly switch from “was measured” to “is analyzed.” That little shift can throw off your flow and confuse your reader.

To avoid this? After you draft a section, go back and highlight your verbs. Ask yourself: “Am I in the right tense for this section?” Stick to past tense for your research actions and present tense for established knowledge. Keep it consistent—it makes you sound way more professional.

Subject-Verb Agreement Errors

You write, “The data shows…” instead of “The data show…” (Yep, “data” is plural.)
Singular subjects need singular verbs; plural subjects need plural verbs.
These mistakes are easy to miss when you’re writing long, complex sentences.

Here’s the kicker: these errors sneak in most when you’re tired. You might type “The list of questions were too long,” not realizing “list” is the actual subject, not “questions.” And in a thesis, you probably write plenty of sentences that twist and turn for miles.

My tip? Break the sentence down. Find the real subject and then check the verb. I started reading my thesis out loud (awkward, yes—but effective), and it helped me catch those slippery agreement errors I’d otherwise skim over.

Incorrect Use of Articles (A, An, The)

You skip articles or use the wrong one—especially if English isn’t your first language.
“A hypothesis” vs. “an hypothesis,” or forgetting “the” before specific nouns.
Missing articles make your writing feel choppy or unclear.

I can’t tell you how many times I’ve read, “This is important step in process…” and cringed a little. It’s easy to overlook because articles feel so small. But in academic writing, they make a big difference in clarity and formality.

Want a trick? Read your sentence slowly and ask: “Am I talking about something general or specific?” Use “the” for specifics and “a/an” for general. Keep a running list of common phrases in your field—like “a result,” “the hypothesis,” “an analysis”—and memorize them.

Misplaced Modifiers

You write, “Running down the hall, the thesis was in my hand.” (Wait, your thesis is running?)
Modifiers need to be placed next to the word they describe.
Misplaced modifiers make your writing unclear or unintentionally funny.

Once, I wrote, “After analyzing the data, the results were clear.” My advisor laughed and asked if the results had done the analyzing. Embarrassing? Yep. But unforgettable.

The fix? Keep modifiers close to the noun they’re modifying. Don’t bury them at the beginning of the sentence unless they clearly match what follows. And when in doubt, rephrase the sentence to make your meaning clearer.

Overuse of Passive Voice

You rely on passive voice to sound “academic”: “The results were analyzed…”
While it’s sometimes appropriate, too much passive voice makes your writing dull.
Active voice is usually clearer and more engaging: “We analyzed the results.”

Here’s the thing. Passive voice isn’t evil—it has its place. But when every sentence reads like a courtroom transcript, readers tune out. Plus, passive voice hides responsibility. Who did what?

When I revised my first draft, I realized almost every sentence started with “It was found that…” or “It is believed…” Ugh. I replaced many of them with active versions: “We found,” “The study shows,” and suddenly everything sounded more confident.

Read your sentences aloud. If they feel flat or vague, try flipping them into active voice.

Confusing “That” vs. “Which”

“That” is restrictive (no commas); “which” is non-restrictive (needs commas).
The wrong one changes the meaning of your sentence—or makes it awkward.

Here’s a quick example: “The theory that explains X is widely accepted” vs. “The theory, which explains X, is widely accepted.” One tells you which theory, the other just adds a detail.

If this always trips you up, don’t stress. You’re not alone. Just remember: if the clause is essential to the meaning, go with “that.” If it’s extra info, use “which” with commas.

Using Ambiguous Pronouns

You write, “It was shown that…” but what’s “it”?
Ambiguous pronouns confuse readers and create vague writing.
Always make sure your pronouns clearly refer to something specific.

You’re probably used to writing fast and assuming people will “get it.” But in a thesis, assumptions are dangerous. I once wrote a paragraph where “this” referred to an entire previous section. No one knew what “this” meant—and honestly, neither did I by the time I revised it.

Be explicit. Instead of “this shows,” write “this result shows” or “this pattern in the data suggests.” The extra words are worth it.

Conclusion

Grammar isn’t just about getting an A on your English paper—it’s about making your ideas clear, credible, and persuasive. When you’re writing your thesis, the last thing you want is for small grammar mistakes to make a big impact on your examiner’s perception.

You’ve worked hard to get here. Don’t let tenses, articles, or rogue modifiers hold you back. By watching for these common grammar mistakes in thesis writing, and editing carefully (or asking someone to review it), you’ll produce work that actually sounds as smart as you are.