Punctuation Rules Explained
Punctuation exists to remove ambiguity, not to decorate a sentence. Most punctuation mistakes come from treating marks as interchangeable pauses rather than as signals with distinct jobs — a comma, a semicolon, and a colon each promise the reader something different about what comes next.
The Comma's Real Jobs
A comma separates items in a list ("apples, oranges, and pears"), sets off an introductory phrase ("After the meeting, we reviewed the notes"), joins two independent clauses along with a coordinating conjunction ("The rain stopped, and the sun came out"), and encloses non-essential information ("My brother, who lives in Denver, is visiting"). That last use is the one most often botched: if the information between the commas could be deleted without changing who or what is being talked about, it needs commas around it; if it identifies which specific person or thing is meant, it doesn't. "Students who skip class often fall behind" needs no commas, because "who skip class" identifies which students; "My sister, who skips class often, fell behind" needs commas, because there's only one sister and the clause is just extra detail.
The Semicolon: Joining Equals
A semicolon joins two independent clauses that are closely related in meaning, without needing a conjunction: "The lab results were inconclusive; we decided to repeat the experiment." Both sides of a semicolon must be able to stand alone as a complete sentence — this is the single most useful test for whether a semicolon is the right choice. A semicolon is also used to separate items in a list when the items themselves already contain commas: "The conference was held in Austin, Texas; Portland, Oregon; and Reno, Nevada."
The Colon: Announcing What Follows
A colon introduces something that explains, lists, or elaborates on what came before: a list ("Bring three things: a notebook, a pencil, and a calculator"), an explanation ("She had one goal: to finish first"), or a quotation. The part before the colon must be able to stand alone as a complete sentence; "The ingredients are: flour, sugar, and eggs" is a common error, since "The ingredients are" isn't itself a complete clause. Removing "are" fixes it: "The ingredients are these: flour, sugar, and eggs," or simply drop the colon entirely: "The ingredients are flour, sugar, and eggs."
Apostrophes for Possession and Contraction
Apostrophes have exactly two jobs: showing possession and marking a contraction. For singular possession, add 's ("the dog's leash"). For plural nouns already ending in s, add only an apostrophe ("the dogs' leash," meaning the leash belonging to multiple dogs). Contractions replace missing letters with an apostrophe exactly where the letters were dropped ("do not" becomes "don't," "they are" becomes "they're"). Apostrophes are never used simply to form a plural — "the 1990's" and "banana's for sale" are both incorrect; the correct forms are "the 1990s" and "bananas for sale."
Quotation Marks and Surrounding Punctuation
In American English, commas and periods go inside closing quotation marks regardless of logic: "She called it a 'minor setback.'" Question marks and exclamation points go inside the quotation marks only if they belong to the quoted material itself; otherwise they go outside: Did she really call it "a minor setback"? British English punctuation conventions differ on some of these points, so consistency within a single document matters more than which convention you pick, if you're writing outside a style guide that specifies one.
Hyphens vs. Dashes
A hyphen joins words into a compound ("well-known author," "twenty-two"). An en dash (–) typically shows a range ("pages 10–15"). An em dash (—) sets off an interruption or emphatic aside within a sentence, functioning somewhat like a more dramatic comma or parenthesis. Mixing these up rarely creates genuine confusion, but consistent use signals careful editing, which matters in formal academic and professional writing.
Summary
Commas, semicolons, and colons each promise the reader a different relationship between ideas, and choosing the wrong one changes what the sentence claims to be doing, not just how it sounds. Apostrophes mark possession and contraction only, never a plain plural. These rules work alongside the sentence-level concerns covered in paragraph structure and unity and the tense choices in verb tenses. Purdue's Online Writing Lab (owl.purdue.edu) offers detailed practice exercises for each punctuation mark covered here.