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Punctuation
Conventions of Standard English
· Topic 1.1
Introduction
A single misplaced comma can change the meaning of a sentence — and cost you points. The ACT knows this, and it tests punctuation on nearly every passage.
Punctuation questions make up roughly 30% of the Conventions of Standard English (CSE) subscore, which itself is about half of your English score. Mastering punctuation alone can raise your English score by 2–4 points.
By the end of this lesson you will be able to:
Can you spot the error? 'The scientist published her findings, however, the journal rejected the manuscript.' By the end of this lesson, you'll know exactly why this sentence fails and how to fix it in under 10 seconds.
The Concept
The Core Rule
Punctuation must serve a grammatical purpose. On the ACT, every comma, semicolon, colon, and dash is either required by a rule, optional for clarity, or flatly wrong. There is no 'sounds right' — only 'follows the rule.'
How the ACT tests this
Underlines a portion of a sentence containing punctuation and asks you to choose the version that is grammatically correct and stylistically appropriate
Offers answer choices that differ only in punctuation type or placement, forcing you to apply specific rules rather than rely on intuition
Offers a tempting NO CHANGE when unnecessary punctuation clutters a sentence — but OMIT (delete the punctuation) is the correct fix. NO CHANGE keeps the error; OMIT removes it
Commas — The Four Legal Uses
Commas are legal in exactly four situations on the ACT. Outside these four, a comma is almost always wrong.
FANBOYS conjunction joining two independent clauses: 'She studied all night, and she passed the exam.'
After an introductory element (word, phrase, or clause): 'After the rain stopped, we went outside.'
Around a non-essential (parenthetical) phrase or clause: 'My brother, who lives in Denver, called yesterday.'
Separating items in a list of three or more: 'She bought apples, oranges, and bananas.'
Semicolons and Colons
Semicolons join two independent clauses without a conjunction. Colons introduce a list, explanation, or elaboration — but only after a complete independent clause.
Semicolon test: replace it with a period. If both halves are complete sentences, the semicolon is legal.
Colon test: everything before the colon must be a complete sentence. 'My favorite sports: tennis and soccer' is WRONG because 'My favorite sports' is not a complete clause.
Never use a semicolon where a colon is needed, and vice versa — the ACT tests this swap deliberately.
Dashes
A single dash emphasizes or introduces; a pair of dashes works like parentheses to set off a non-essential interruption. The ACT frequently tests whether you use one dash or two.
One dash before an explanation or list at the end: 'She had one goal — to win the championship.'
Two dashes around a mid-sentence interruption: 'The experiment — which lasted three years — yielded surprising results.'
Never mix a dash with a comma to frame the same interruption.
Apostrophes
Apostrophes signal possession or contraction. The ACT tests the its/it's and their/there/they're distinctions repeatedly.
Possessive nouns: add 's for singular (dog's collar); add s' for regular plurals (dogs' collars).
it's = it is (contraction); its = belonging to it (possessive pronoun — NO apostrophe).
Possessive pronouns NEVER take apostrophes: his, hers, its, ours, yours, theirs.
Your strategy
1
Step 1 — Identify what punctuation mark is being tested by comparing the answer choices. If all choices differ only in punctuation, apply punctuation rules, not style.
2
Step 2 — For commas, check which of the four legal uses applies. If none fits, eliminate the comma.
3
Step 3 — For semicolons and colons, test each side: semicolons need two independent clauses; colons need a complete clause before them.
4
Step 4 — When OMIT or NO CHANGE is an option, ask: 'Does this punctuation serve a grammatical purpose?' If not, OMIT is likely correct.
Worked Examples
Easy
Example 1
Comma Splice Trap — The Comma In A Looks Natural Because It Mirrors Spoken Rhythm, But It Violates The Rule That A Comma Alone Cannot Join Two Independent Clauses.
Marine biologists have long studied the migration patterns of humpback whales. These mammals travel thousands of miles each year, [they navigate using the Earth's magnetic field]. The precision of their internal compass rivals any technology humans have developed.
Which choice best punctuates the underlined portion?
A.
year, they navigate
B.
year; they navigate (Correct answer)
C.
year they navigate
D.
year: they navigate
Step 1
Identify the issue: two clauses are joined at the underlined portion. Check whether each is independent.
Step 2
'These mammals travel thousands of miles each year' — complete sentence. 'they navigate using the Earth's magnetic field' — complete sentence.
Step 3
Two independent clauses joined without a conjunction need a semicolon, not a comma (comma splice) and not nothing (run-on).
Step 4
A colon requires the first clause to introduce what follows — here, the second clause is not an explanation of 'year.' Semicolon wins.
Correct answer: B
Why B is correct
Correct — semicolon legally joins two independent clauses.
Why other options are wrong
A: Comma splice — a comma alone cannot join two independent clauses.
C: Run-on sentence — no punctuation between two independent clauses.
D: Colon misuse — 'travel thousands of miles each year' does not introduce or explain 'they navigate using the Earth's magnetic field' as a list or elaboration.
⚠ Trap: Comma splice trap — the comma in A looks natural because it mirrors spoken rhythm, but it violates the rule that a comma alone cannot join two independent clauses.
Medium
Example 2
Punctuation Mismatch Trap — Answer A Uses A Dash On One Side Of An Interruption While The Other Side Uses A Comma, Which Is Never Acceptable.
The Baroque period produced some of history's most technically demanding compositions. Johann Sebastian Bach, [whose mastery of counterpoint — was unmatched,] wrote over a thousand works before his death in 1750. Many of those pieces remain central to the classical canon today.
Which choice best punctuates the underlined portion?
A.
whose mastery of counterpoint — was unmatched,
B.
whose mastery of counterpoint was unmatched, (Correct answer)
C.
whose mastery of counterpoint, was unmatched,
D.
whose mastery of counterpoint — was unmatched —
Step 1
The underlined clause 'whose mastery of counterpoint was unmatched' is a non-essential relative clause modifying Bach.
Step 2
Non-essential clauses should be set off with a matching pair of commas — one before 'whose' (already present as 'Bach,') and one after 'unmatched.'
Step 3
A dash is introduced mid-clause in A and D, breaking the phrase illogically — there is no reason to interrupt 'mastery of counterpoint' with a dash.
Step 4
C adds a comma between 'counterpoint' and 'was,' separating the subject of the relative clause from its verb — illegal comma placement.
Correct answer: B
Why B is correct
Correct — the relative clause flows cleanly, and the closing comma matches the opening comma after 'Bach.'
Why other options are wrong
A: Dash before 'was' has no grammatical purpose and disrupts the relative clause structure.
C: Illegal comma between the subject ('mastery') and verb ('was') of the relative clause.
D: Two dashes would be correct only if the entire interrupting phrase were between them, but the opening comma after 'Bach' creates a mismatch.
⚠ Trap: Punctuation mismatch trap — answer A uses a dash on one side of an interruption while the other side uses a comma, which is never acceptable.
Hard
Example 3
Semicolon Vs. Colon Confusion — Students Who Know Semicolons Join Independent Clauses Pick B, Missing That The Colon's Explanatory Relationship Is The More Precise And ACT-preferred Choice.
Urban planners in the 1960s believed that elevated highways would solve traffic congestion. The plan had one fatal flaw[: it severed neighborhoods, displaced thousands of residents, and created corridors of noise and pollution that persisted for decades]. Later generations spent billions demolishing the very structures their predecessors had celebrated.
Which choice best punctuates the underlined portion?
A.
flaw, it severed
B.
flaw; it severed
C.
flaw: it severed (Correct answer)
D.
flaw — it severed
Step 1
The first clause 'The plan had one fatal flaw' is complete and ends with a noun — 'flaw' — that signals something will be explained.
Step 2
What follows is a direct elaboration of what the flaw was. This introduction-then-explanation structure is the textbook colon use case.
Step 3
A semicolon (B) would work grammatically but does not capture the explanatory relationship — the ACT prefers the most precise punctuation.
Step 4
A dash (D) is also grammatically acceptable here, but when both a colon and a dash are offered and the setup is a classic 'one thing: here it is' structure, the colon is the ACT-preferred answer.
Correct answer: C
Why C is correct
Correct — the independent clause directly introduces its explanation, the defining use case for a colon.
Why other options are wrong
A: Comma splice — two independent clauses cannot be joined by a comma alone.
B: Grammatically legal but imprecise — a semicolon implies coordinate equal clauses, missing the 'flaw → explanation' relationship.
D: Grammatically acceptable but the colon is more precise and conventional for a 'one fatal flaw: here it is' construction.
⚠ Trap: Semicolon vs. colon confusion — students who know semicolons join independent clauses pick B, missing that the colon's explanatory relationship is the more precise and ACT-preferred choice.
Strategy Tips
Always compare answer choices first — if they differ only in punctuation, apply the rule mechanically rather than reading aloud.
Test semicolons by substituting a period: if both sides become complete sentences, the semicolon is legal.
For colons, cover everything after the colon and ask: is what remains a complete sentence? If not, the colon is wrong.
For dashes, count them: one dash at the end introduces; two dashes in the middle replace parentheses. Never mix one dash with one comma around the same interruption.
When OMIT is an option, always ask whether the punctuation serves a grammatical function. Extra commas between a subject and its verb, or between a verb and its object, are almost always wrong.
Common pitfalls
Comma before 'which' or 'who': only use a comma when the clause is non-essential (can be removed without changing meaning). 'The report that she wrote' — no comma; 'The report, which took months, was excellent' — commas required.
Apostrophe on possessive pronouns: its, hers, theirs, ours, yours NEVER take apostrophes. Only nouns take possessive apostrophes.
Colon after an incomplete clause: 'Her hobbies include: reading and hiking' is WRONG because 'Her hobbies include' is not a complete sentence standing alone.
Punctuation questions should take 20–30 seconds each. If you find yourself re-reading the passage more than twice, you are over-thinking it — apply the rule to just the underlined portion and the immediate surrounding clause.
Summary
Commas have exactly four legal uses on the ACT; any comma outside those four contexts is almost certainly wrong.
Semicolons join two independent clauses; colons introduce an explanation or list after a complete clause — knowing the difference is worth several points.
Apostrophes never appear on possessive pronouns (its, theirs, yours), and the colon/dash distinction comes down to explanation vs. dramatic emphasis.
Find any paragraph in a textbook or news article. Rewrite every sentence by deliberately introducing one punctuation error, then swap with a study partner and identify each other's errors using only the four comma rules, the semicolon period-test, and the colon complete-clause test.