Most Common SAT Grammar Mistakes and How to Avoid Them

The SAT Writing & Language section tests the same grammar mistakes over and over. Understanding which errors appear most often — and exactly why they're wrong — lets you recognize them instantly rather than guessing. Here are the mistakes that cost students the most points, with clear explanations and fixes.

Mistake 1: Comma Splices

A comma splice joins two independent clauses with only a comma, which is incorrect.

Wrong: "The experiment failed, the team revised their hypothesis."
Fixed options:

• "The experiment failed; the team revised their hypothesis." (semicolon)
• "The experiment failed, so the team revised their hypothesis." (comma + FANBOYS)
• "Because the experiment failed, the team revised their hypothesis." (subordinating conjunction)

Comma splices are the most common Writing section error. Train yourself to check: does the word after the comma start a new complete thought? If yes, a comma alone won't do.

Mistake 2: Incorrect Apostrophe Use

Apostrophes show possession or contraction — never plurals.

• Wrong: "The student's went to class." — apostrophe doesn't belong here (just a plural)
• Wrong: "Its a difficult problem." — "its" is possessive; "it's" = "it is"
• Right: "The student's essay was excellent." — "student's" shows possession

Quick check: if you can replace the word with "it is" and the sentence still makes sense, use "it's." If not, use "its."

Mistake 3: Faulty Parallelism

Items in a series must share the same grammatical structure. Students often write parallel lists correctly in simple sentences but fail in more complex constructions.

Wrong: "The program teaches students to code, designing algorithms, and how they can manage projects."
Right: "The program teaches students to code, to design algorithms, and to manage projects."

Look for lists with three or more items and verify they're all the same form: all infinitives, all gerunds, all noun phrases, or all parallel clauses.

Mistake 4: Subject-Verb Disagreement Across Distance

The SAT deliberately inserts long phrases between subjects and verbs to trigger agreement errors.

Wrong: "The collection of rare manuscripts, many of which date back centuries, were donated to the library."
Right: "The collection... was donated." — "collection" is the subject, not "manuscripts."

Fix: cross out the intervening phrase mentally and match verb to bare subject.

Mistake 5: Redundancy

The SAT Writing section frequently tests concision — eliminating unnecessary repetition.

• "advance forward" → "advance" (forward is implied)
• "past history" → "history" (history is always past)
• "the reason is because" → "the reason is that"
• "end result" → "result"

When a Writing question offers a shorter option that preserves the same meaning, the shorter option is almost always correct.

Mistake 6: Wrong Transition Words

Many students pick transition words by feeling rather than by logical relationship. The SAT tests this directly.

• Contrast: however, although, despite, nevertheless, yet
• Causation: therefore, thus, consequently, as a result
• Addition: furthermore, moreover, in addition, also
• Example: for instance, for example, specifically

Before selecting a transition word, identify the logical relationship between the two sentences being connected. Does the second sentence contrast, add to, follow from, or exemplify the first? Then match the transition to that relationship.

How to Avoid These Mistakes Under Pressure

The SAT Writing section gives you roughly 35 seconds per question. Under that pressure, errors slip through because you're pattern-matching by ear rather than by rule. The fix: for each question type, identify the rule before reading the answer choices. "This is a comma question. Is there a comma splice?" forces you to apply a rule rather than guess.

After three practice sessions of naming rules before answering, the habit becomes automatic — and your error rate on these question types drops measurably.