Punctuation Rules

The method, step by step

1. 1

   Test whether each part could stand alone

   'It was raining' + 'we stayed inside' → both complete

   This test decides most punctuation questions. Two parts that could each be a sentence on their own need a full stop, semicolon, or comma plus conjunction between them — never a comma by itself.

2. 2

   Join complete sentences correctly

   full stop · semicolon · comma + and/but/or/so · colon

   Four legitimate options, each with a different effect. A semicolon links closely related ideas; a colon signals that what follows explains or lists; a comma requires a conjunction alongside it.

3. 3

   Use commas to separate, not to join

   lists, after an introductory phrase, around extra information

   Commas mark boundaries within a sentence rather than between sentences. Setting off non-essential information requires a pair — one before and one after — and dropping the second is a frequent error.

4. 4

   Use apostrophes for possession or omission

   the dog's bone (possession) · don't (omission)

   Apostrophes never make a plural. 'The dog's are barking' is wrong; 'the dogs are barking' is correct. Plural possession puts the apostrophe after the s: the dogs' bones.

5. 5

   Remember that its and it's are the exception

   its = belonging to it · it's = it is

   This reverses the usual rule because 'its' is a possessive pronoun, like 'his' and 'hers', which never take apostrophes. Expanding 'it's' to 'it is' and checking the sentence still works settles it every time.

The most common error in English

Joining two complete sentences with only a comma — 'It was raining, we stayed inside' — is a comma splice, and it is the single most frequent punctuation error in student writing and the most heavily tested on the SAT.

The fix is any of four things: a full stop, a semicolon, a comma with a conjunction, or a colon if the second part explains the first. Which one to choose is a matter of style; using none of them is an error.

• Two complete sentences need more than a comma
• Semicolon: closely related complete sentences
• Colon: what follows explains or lists
• Non-essential information takes a pair of commas
• Apostrophes never make plurals
• its = possession, it's = it is

Semicolons and colons do different jobs

A semicolon joins two complete sentences whose ideas are closely linked — it is a softer full stop. A colon introduces something: an explanation, a list, or an example. What follows a colon does not have to be a complete sentence, though what precedes it does.

The practical distinction is that a semicolon balances two equal halves, while a colon points forward from the first half to the second. Students who use them interchangeably tend to overuse the semicolon.

Apostrophes do two jobs and no others

They mark possession — the girl's book — or omitted letters — can't, we'll, it's. That is the complete list. They never indicate a plural, despite the widespread greengrocer's usage on signs.

For plural possession, add the apostrophe after the existing s: the students' results. For singular nouns ending in s, both James's and James' are accepted, though consistency within a piece matters more than the choice.

How we teach punctuation

We teach the stand-alone test first, because it decides the majority of punctuation questions and it can be applied to any sentence, familiar or not. Rules learned as lists of cases fail on unfamiliar structures.

For its and it's we teach the expansion check rather than the rule. Substituting 'it is' and seeing whether the sentence still makes sense is faster than recalling why the possessive is the exception.