Why AI can still get grammar wrong

Modern AI tools are trained on huge amounts of text, so they’ve seen millions of correct sentences. That’s why they usually produce grammatically acceptable writing.

But “usually correct” isn’t the same as “always right”. AI still:

  • Guesses based on patterns instead of understanding grammar rules.
  • Gets confused when sentences are long or complex.
  • Misses subtle issues with word choice and style.

The result: the text looks polished at first glance, but there are small slips your teacher, editor, or reader will notice.

Good news: Once you know the common mistakes to look for, you can fix most of them quickly—especially with the help of a dedicated grammar tool like the one built into OpenHumanizer.

1. Confusing similar words (“your” vs “you’re”, etc.)

AI is better than many humans at basic word choice, but it still occasionally confuses words that sound similar or are used in similar contexts.

Some classic examples include:

  • your / you’re – “Your going to love this” instead of “You’re going to love this”.
  • its / it’s – “Its a good idea” instead of “It’s a good idea”.
  • there / their / they’re – Especially in fast, informal outputs.
  • then / than – “It was better then before” instead of “better than before”.

These errors usually show up when the AI is generating text quickly or when it has tried to match a more casual style.

How to fix this

  • Do a quick manual scan focusing just on these common word pairs.
  • Run the text through the grammar checker in OpenHumanizer to catch obvious mix-ups.
  • Read sentences out loud—often your ear will catch “Your going” as wrong even if your eyes skip over it.

2. Missing or extra commas

Commas are tricky even for human writers. AI often:

  • Skips commas in long introductory phrases.
  • Adds extra commas where they aren’t needed.
  • Is inconsistent—using a comma in one similar sentence but not in another.

For example:

  • “In addition to this many students feel…” (missing comma after “this”).
  • “The main point is, that students need support.” (unnecessary comma before “that”).

How to fix this

  • Look for sentences that feel “too long” when read out loud—these often need a comma or a full stop.
  • Use the grammar checker to highlight punctuation issues, then review each suggestion to make sure the meaning still makes sense.
  • In academic writing, err on the side of clarity: shorter sentences with fewer commas are often easier to read.

3. Awkward prepositions (“in”, “at”, “for”, etc.)

Prepositions are small words, but they matter a lot. AI sometimes chooses the wrong one because it’s guessing based on similar sentences rather than understanding the exact phrase.

You might see things like:

  • “Interested on this topic” instead of “interested in this topic”.
  • “Discussed about the issue” instead of “discussed the issue”.
  • “On the other hand for students” where “for students” doesn’t quite fit the structure.

How to fix this

  • Slowly reread sentences with “in”, “on”, “at”, “about”, “for”, and “to”.
  • If something sounds slightly strange, search that exact phrase in your memory—have you seen it in good writing before?
  • Use the grammar tool to catch the most obvious cases, then rely on your own sense of natural phrasing for the rest.

4. Overusing certain phrases

While this isn’t always a “grammar error”, it does affect how polished and human your writing feels. AI often repeats:

  • “In conclusion”
  • “Furthermore”
  • “On the other hand”
  • “It is important to note that”

Using these occasionally is fine. Using them in almost every paragraph makes the writing feel mechanical and predictable.

How to fix this

  • Search your text for phrases like “in conclusion” and see how many times they appear.
  • Replace some of them with simpler, more natural transitions—or remove them entirely.
  • Use OpenHumanizer with a different tone or style to reduce repeated formal connectors.

5. Slightly wrong verb tenses

AI is usually good with basic tense, but it can slip when:

  • Switching between past and present in the same paragraph.
  • Describing events that happened in a sequence.
  • Trying to match your tense choices from earlier sentences.

For example, you might see:

  • “Last year, the school introduces a new policy…” (should be “introduced”).
  • “Students were feeling stressed and still feel worried today.” (tense mixing).

How to fix this

  • Choose a main tense for the piece (usually past for stories, present for general facts).
  • Reread each paragraph and check that the tense is consistent unless you deliberately change it.
  • Use grammar checking as a second layer, especially for long essays.

6. Agreement errors (singular/plural, subject/verb)

Sometimes AI loses track of whether it’s talking about one thing or many things. This leads to:

  • “The main problem are the stress and pressure.”
  • “This data shows that students is struggling.”

These mistakes often happen when:

  • AI edits a sentence mid-generation and doesn’t fully adjust the grammar.
  • Long noun phrases separate the subject and verb, making it easy to get confused.

How to fix this

  • Identify the core subject (“problem”, “students”, “data”) and check if the verb matches.
  • Simplify long subjects into shorter ones when possible.
  • Use the grammar checker to flag “agreement” issues you might miss by eye.

7. Slightly unnatural word order

AI sometimes produces sentences that are technically correct but sound slightly odd:

  • “Very important it is to consider…”
  • “Rarely students do think about this deeply.”

These are the kinds of sentences where a reader might pause and think, “Hmm, that’s not how I’d say it.”

How to fix this

  • Read the text out loud (or in your head as if you were speaking to a friend).
  • When a sentence feels stiff, try reordering it: “It is very important to consider…”
  • Use OpenHumanizer with a more casual or balanced tone to smooth out unnatural phrasing.

Using OpenHumanizer’s grammar tools effectively

OpenHumanizer includes a built-in grammar correction feature designed to catch many of the issues listed above. To get the most out of it:

  • Paste your text into the grammar correction section (or let it auto-fill from the humanized output).
  • Choose the right level: standard or advanced is ideal when you want both error correction and smoother phrasing.
  • Pick US or UK English to match your assignment or audience.
  • Review the result with your own eyes instead of accepting every change blindly.
Important: Grammar tools—including the one in OpenHumanizer—are there to support you, not to replace your judgement. If a “correction” changes the meaning in a way you don’t like, change it back.

A quick review checklist

Before submitting AI-assisted writing, run through this quick checklist:

  • ✔ Have I checked for basic word mix-ups (your/you’re, its/it’s, there/their/they’re)?
  • ✔ Do the commas feel natural when I read the text out loud?
  • ✔ Are the prepositions (“in”, “on”, “at”, “about”) used in ways that sound normal?
  • ✔ Did I remove repeated connector phrases like “Furthermore” in every paragraph?
  • ✔ Are verb tenses consistent within each section?
  • ✔ Does each subject match its verb in number (singular/plural)?

Final thoughts: AI + grammar tools + your eyes

AI has made it easier than ever to produce clean, fluent English—but that doesn’t mean you can skip grammar completely. In fact, small errors matter even more when the rest of the text looks polished.

The best approach is layered:

  • Use AI to generate or refine your draft.
  • Run it through OpenHumanizer to improve tone and flow.
  • Use the grammar correction tool to catch most technical issues.
  • Do one final human pass where you focus on meaning, clarity, and natural sound.
Try this exercise: Take a paragraph of AI-generated text, run it through the OpenHumanizer tool, then send it to the grammar checker. Compare the original and final versions side by side and mark every change you agree with. You’ll quickly build an eye for the kinds of mistakes AI tends to make.