Common Grammar Mistakes KS3 Students Make (and How to Fix Them)

The most common grammar mistakes at KS3 fall into five categories: apostrophe errors, comma splices, inconsistent tense, vague pronoun reference, and run-on sentences. Each one has a clear rule that can be learned, practised and eliminated from your writing before assessments.

Why grammar matters in KS3 English

Grammar, punctuation and spelling — assessed together as SPaG — carry explicit marks in most KS3 English assessments and carry a dedicated mark scheme at GCSE. More than that, weak grammar makes even strong ideas harder for a reader to follow. A persuasive argument undermined by apostrophe errors or tense confusion signals carelessness, which reduces the overall impression the writing creates.

The DfE's KS3 English curriculum requires students to write with "accurate spelling and punctuation" and to use "a range of sentence structures for clarity, purpose and effect." These are not passive expectations — they are assessed competences that improve with deliberate practice.

The good news: grammar errors are among the most fixable issues in student writing. Unlike analytical insight, which takes time to develop, a student who learns and practises the five rules below can eliminate most common errors within a few weeks.

Mistake 1 — Apostrophe errors

Apostrophes are misused in two distinct ways that require separate understanding.

Possession

An apostrophe + s ('s) shows that something belongs to a person or thing:

the teacher's question (one teacher)

the students' essays (multiple students — apostrophe after the s)

the child's book / the children's books (irregular plural — apostrophe before the s)

Common error: Adding an apostrophe to possessive pronouns that do not need one.

Incorrect | Correct
it's colour | its colour
your's | yours
their's | theirs

Rule to memorise: its, yours, his, hers, theirs, ours — these possessive pronouns never take an apostrophe.

Contractions

An apostrophe marks where letters have been removed in a contraction:

it's = it is (this is the only time it's takes an apostrophe)

don't = do not

they're = they are

Common error: Confusing it's (contraction: it is) with its (possessive: belonging to it). One test: expand the contraction. "The dog wagged it's tail" → "The dog wagged it is tail" — which makes no sense, so its (possessive) is correct.

Mistake 2 — The comma splice

A comma splice is joining two independent main clauses with a comma alone, without a conjunction. It is one of the most common errors at KS3 and GCSE.

Comma splice (incorrect): She studied hard for the test, she felt confident on the morning.

Both halves are complete sentences — "She studied hard for the test" and "she felt confident on the morning." A comma alone is not strong enough to join them.

Three correct alternatives:

Full stop: She studied hard for the test. She felt confident on the morning.

Conjunction: She studied hard for the test, so she felt confident on the morning.

Semicolon: She studied hard for the test; she felt confident on the morning. (Use a semicolon when the two clauses are closely related in meaning.)

A quick test: read each half of the sentence in isolation. If both halves are complete sentences, a comma alone cannot join them.

Mistake 3 — Inconsistent tense

Tense should remain consistent within an essay or analytical paragraph unless there is a deliberate reason to shift (for example, moving from narrative past tense to commenting on an author's technique in present tense).

Incorrect (shifting tense): The writer creates tension through short sentences. In the next paragraph, she described the setting in detail.

Creates is present tense; described is past tense. When writing about a text, literary convention is to use the present tense throughout: the writer "creates," the character "feels," the imagery "suggests."

Correct: The writer creates tension through short sentences. In the next paragraph, she describes the setting in detail.

Incorrect (in narrative): The boy walked into the room. He looks around and sees a stranger. He froze.

Correct (past tense consistently): The boy walked into the room. He looked around and saw a stranger. He froze.

Set a simple self-editing rule: at the end of every paragraph, check the tense of the first and last verb. If they do not match, identify and correct the inconsistency.

Mistake 4 — Vague pronoun reference

Pronoun reference errors occur when it is unclear which noun a pronoun refers to. They make writing ambiguous and confusing.

Vague: The writer uses pathetic fallacy. This creates tension. — What does "this" refer to? The use of pathetic fallacy, the pathetic fallacy itself, or the entire paragraph? The reference is unclear.

Clear: The writer's use of pathetic fallacy creates tension by aligning the weather with the character's emotional state.

Vague: When Sarah told her teacher about the problem, she was very upset. — Who was upset, Sarah or the teacher?

Clear: When Sarah told her teacher about the problem, Sarah was very upset.

Rule: Every time you write "this," "it," "they" or "she/he," ask: can a reader identify exactly what noun this pronoun replaces? If not, name the noun instead.

Mistake 5 — Run-on sentences

A run-on sentence connects multiple clauses without adequate punctuation, making the meaning hard to follow.

Run-on: The character enters the room she feels afraid the atmosphere is dark and the writer uses several techniques to build suspense including short sentences and sound imagery.

This contains at least four separate ideas. A reader cannot track them when strung together without punctuation.

Corrected (with appropriate punctuation and structure): The character enters the room feeling afraid. The atmosphere is dark, and the writer uses several techniques to build this suspense — short sentences and sound imagery are the most notable.

Students who write in long, unpunctuated sentences are often thinking faster than they are punctuating. Slowing down and reading each sentence aloud is the best cure: a sentence that requires a breath in the middle usually needs a full stop or semicolon at that point.

A self-editing checklist for KS3 writing

After completing a piece of writing, read it through specifically looking for:

Every apostrophe — is each one marking either a contraction or possession?

Every comma — is it ever joining two main clauses alone without a conjunction?

The tense of verbs — is the tense consistent throughout each paragraph?

Every "this," "it," "they" — does each one have a clear noun it refers to?

Long sentences — does each one remain readable when spoken aloud?

One read-through dedicated to SPaG — separate from the read-through checking ideas and structure — will catch most of these errors.

Frequently asked questions

What is the most common grammar mistake in KS3 English writing?

Apostrophe errors are the most frequent, particularly confusing its (possessive) with it's (contraction for "it is"), and incorrectly adding apostrophes to possessive pronouns such as yours and theirs. Comma splices — joining two main clauses with a comma instead of a conjunction or full stop — are the second most common error.

How do I know when to use a semicolon at KS3?

A semicolon joins two independent main clauses that are closely related in meaning, without a conjunction. Think of it as stronger than a comma but weaker than a full stop. At KS3, using semicolons correctly is a mark of sophisticated punctuation control. If you are unsure whether a semicolon is correct, use a full stop — a correct full stop is better than an uncertain semicolon.

What is a comma splice and how do I fix it?

A comma splice is using a comma alone to join two complete sentences: She revised carefully, she felt confident. Fix it with a full stop, a conjunction (she revised carefully, so she felt confident), or a semicolon (she revised carefully; she felt confident). To identify one, test whether both halves of the sentence could stand alone — if yes, a comma alone is not sufficient.

Does tense matter in English literature essays at KS3?

Yes. The literary convention is to write about texts in the present tense: the writer "creates," the character "feels," the imagery "suggests." This applies even when discussing events that happened in a past-tense narrative. Switching between past and present tense within a literature essay is one of the most commonly penalised inconsistencies in KS3 and GCSE marking.

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