Punctuation inside a sentence

Once the sentence boundary is settled, the remaining punctuation questions are about what happens inside one sentence.

#The default: no comma

The most useful bias on this whole domain. A comma needs a reason. If you can't name the rule, the answer is no comma. Test writers rely on students inserting commas where they'd pause when speaking, and pauses aren't a rule.

Specifically, never put a comma:

• Between a subject and its verb. ✗ The results of the experiment, were inconclusive.
• Between a verb and its object. ✗ She argued, that the model was flawed.
• Before a that-clause. ✗ The evidence suggests, that the pattern is real.

#Non-essential elements

Information you could delete without changing what the sentence identifies gets paired punctuation — two commas, two dashes, or two parentheses. Both ends, and matching:

• ✓ Marie Tharp, a geologist, mapped the ocean floor.
• ✓ Marie Tharp — a geologist — mapped the ocean floor.
• ✗ Marie Tharp, a geologist — mapped the ocean floor.

The mismatched pair is a favourite wrong answer because each mark looks fine on its own.

Essential information gets no punctuation: The geologist who mapped the ocean floor was Marie Tharp. Delete who mapped the ocean floor and you no longer know which geologist — so it stays unpunctuated.

#The colon

One rule: a complete sentence must come before it. After it can be anything — a list, a phrase, one word, another sentence.

• ✓ She needed three things: time, funding, and quiet.
• ✗ She needed: time, funding, and quiet. (She needed isn't a sentence.)

If the test offers a colon and the left side isn't independent, eliminate it without reading further.

#The dash

Works like a colon (one dash, introducing) or like paired commas (two dashes, enclosing). What it can't do is appear once in the middle of a phrase that also uses a comma at the other end.

#Apostrophes

Three questions, in order:

1. Plural or possessive? Plural takes no apostrophe. The studies showed — no apostrophe, ever.
2. If possessive: singular or plural owner? the scientist's data (one) vs the scientists' data (several).
3. Is it its or it's? It's is only ever "it is" or "it has". The possessive its has no apostrophe. Read it aloud as "it is" — if that's wrong, drop the apostrophe.

And whose vs who's works identically.

#Lists

Items are separated by commas. If any item already contains a comma, the separators become semicolons: Delegates came from Lima, Peru; Quito, Ecuador; and Bogotá, Colombia.