The Core Idea
A Small Set of Errors, a Large Share of Lost Points
Four specific grammar errors show up in student writing far more often than any others, and each one has a clean, learnable fix: run-on sentences, comma splices, subject-verb agreement errors, and Oxford comma omissions. Learning to spot these four reliably eliminates a disproportionate share of the grammar errors that cost points on otherwise strong essays.
A run-on sentence occurs when two independent clauses (complete sentences on their own) are joined WITHOUT proper punctuation. A comma splice is the closely-related error of joining two independent clauses with ONLY a comma, which isn't strong enough punctuation on its own to do that job.
💡 Memory Trick
Picture two complete sentences as two cars that need a proper coupling to be safely joined together. A single comma is like a thin piece of string trying to tow one car behind another — it looks like it might work, but it will snap under any real weight (that's a comma splice). A period, semicolon, or a comma PLUS a coordinating conjunction (FANBOYS: For, And, Nor, But, Or, Yet, So) is like an actual tow hitch — strong enough to properly join the two cars (correct sentences) together.
The Four Errors and Their Fixes
Run-ons, Comma Splices, Agreement, and the Oxford Comma
1
Run-On Sentences
'I went to the store I bought milk.' Two independent clauses with NO punctuation joining them. Fix: split into two sentences ('I went to the store. I bought milk.'), use a semicolon ('I went to the store; I bought milk.'), or add a comma plus a FANBOYS conjunction ('I went to the store, and I bought milk.').
2
Comma Splices
The same problem, but with a comma alone incorrectly trying to join the two independent clauses — the fix is identical to the run-on fix: split into two sentences, use a semicolon, or add a coordinating conjunction after the comma.
3
Subject-Verb Agreement
A singular subject needs a singular verb, and a plural subject needs a plural verb — errors often creep in when other words sit between the subject and its verb, tempting you to match the verb to the nearest noun instead of the actual subject.
4
The Oxford Comma
The final comma before 'and' in a list of three or more items, used specifically to prevent ambiguity — most style guides and professors require it. Compare: 'I thanked my parents, Batman and Robin' (ambiguous — are your parents Batman and Robin?) versus 'I thanked my parents, Batman, and Robin' (clearly three separate items).
Two More Frequent Trouble Spots
Its vs. It's, and Active vs. Passive Voice
'Its' (no apostrophe) is possessive ('the dog licked its paw'); 'it's' (with apostrophe) is the contraction for 'it is' ('it's raining'). This confusion is worth memorizing precisely because an apostrophe NEVER indicates possession for pronouns — 'yours,' 'hers,' 'theirs,' and 'its' all skip the apostrophe entirely, unlike regular nouns.
ACTIVE voice ('The scientist discovered the cure') states who performed the action directly; PASSIVE voice ('The cure was discovered by the scientist') restructures the sentence so the action's recipient comes first. Active voice is generally clearer and more direct in academic writing, though passive voice has a legitimate place when the agent performing the action is unknown or genuinely unimportant to the sentence's point.
🖥️ Applied Scenario
A student's draft includes the sentence: 'The committee, along with several outside consultants, are reviewing the proposal, however they have not reached a decision.'
1
You identify a subject-verb agreement error: the actual subject is 'The committee' (singular), not 'consultants' (the nearest noun) — the correct verb is 'is,' not 'are.'
2
You identify a comma splice: 'the proposal, however they have not reached a decision' joins two independent clauses with only a comma before 'however,' which isn't strong enough punctuation for this job.
3
You fix the comma splice by adding a semicolon before 'however' (a transitional word like 'however' typically needs a semicolon before it and a comma after it when joining two independent clauses this way).
4
Conclusion: the corrected sentence reads 'The committee, along with several outside consultants, is reviewing the proposal; however, they have not reached a decision' — fixing both the subject-verb agreement error and the comma splice in one pass.
📌 Exam Application
Exam and assignment rubrics frequently ask you to identify and correct run-on sentences, comma splices, and subject-verb agreement errors within sample sentences. You may also be asked to explain when the Oxford comma prevents genuine ambiguity, or to convert a sentence between active and passive voice.
⚠️ Most Common Common Grammar Errors Mistakes
The most common mistake is matching a verb to the nearest noun rather than the actual grammatical subject — in a sentence like 'The committee, along with several consultants, is reviewing,' the subject is still 'committee' (singular), regardless of the plural noun sitting closer to the verb. Another frequent error is assuming a comma alone can join two independent clauses — a comma splice needs either a stronger punctuation mark (period, semicolon) or an added coordinating conjunction, since a comma by itself isn't sufficient.
✓ Quick Self-Test
Given a sample sentence, can you identify whether it contains a run-on, a comma splice, or a subject-verb agreement error, and correct it? Can you explain why 'its' never takes an apostrophe when used possessively?
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