A comma in English marks structure rather than breath. Placing them where you would pause if you read the sentence aloud is the single habit behind most punctuation errors in second-language writing, and it produces both faults at once: commas where the grammar wants none, and none where the grammar wants one.
The comma has four jobs and that is the complete list. It separates items in a series. It closes an introductory element before the main clause. It encloses an interruption, with a comma on each side. And it goes before and, but or so when what follows is a complete sentence rather than a continuation.
Eight marks and what each one is for
- the semicolon joins two full sentences that are closely related, and nothing else. If either half cannot stand alone, it is the wrong mark.
- the colon introduces a list, an explanation or a quotation, and what comes before it must be a complete sentence.
- the apostrophe marks possession or omission, never a plural. A sign advertising apple's is the standard example.
- its and it's. Possessive and it is, and this one pair accounts for a large share of all apostrophe errors.
- the Oxford comma is optional before the final and in a list, required by some style guides, and worth using whenever leaving it out creates ambiguity.
- dashes are a pair when they interrupt a sentence, like brackets, and a single one at the end works differently.
- a comma before which, not before that, which is the punctuation half of the relative clause rule.
- quotation marks and the full stop. British style puts the stop outside unless it belongs to the quotation; American style puts it inside regardless.
The one error worth automating
Joining two complete sentences with a comma is called a comma splice, and it is the most common punctuation error in second-language English by a wide margin. The report was late, the committee had already met. Both halves stand alone, so the comma cannot hold them.
The test needs no grammatical knowledge. Cover everything after the comma and ask whether what is left is a sentence. Cover everything before it and ask the same. If both answers are yes, the comma is wrong, and there are three legitimate repairs: a full stop, a semicolon, or a comma with and, but or so added after it.
Knowing that one test settles more punctuation than any other rule on this page, because the splice accounts for a large share of what gets marked and it is entirely mechanical.
The semicolon has exactly one job
It joins two sentences that could each stand alone and that belong together closely enough to share a line. That is all. It is not a heavy comma, it does not introduce a list, and it is not a mark of sophistication, which is the belief that causes most of its misuse.
The colon is the one that introduces: a list, an explanation, or an example, and what comes before a colon has to be a complete sentence. That single condition prevents nearly every colon error anybody makes.
Where it stops: punctuating by ear
The stall is reading the sentence in your head and putting a comma where you would breathe, and it survives because it works often enough. English commas do frequently fall near natural pauses, so a writer using this method is right perhaps two thirds of the time, and two thirds is enough to prevent the method being questioned.
The third that is wrong is not random, either. Long subjects invite a breath before the verb, so writers separate a subject from its verb with a comma, which English never allows. And two short sentences on the same subject feel like one breath, which is exactly where the splice comes from. The ear is producing the same two errors over and over, and no amount of care will fix a method whose premise is wrong.
The apostrophe, which is two rules and one exception
It marks possession and it marks a contraction. It never marks a plural, in any circumstance, including dates, abbreviations and family names on a sign.
The exception that catches everybody is its against it's, and it is genuinely irregular. It's is only ever it is or it has. The possessive is its with no apostrophe, which follows his and hers, and those take none either. Reading it that way makes it a pattern rather than an exception, which is the only thing that makes it stick.
A reference with examples rather than rules
English Grammar Today, free to read on the Cambridge Dictionary site, is the practical reference here because its entries are built on real examples rather than on invented ones, so the punctuation is shown in sentences somebody actually wrote. It replaced the punctuation chapter at the back of a coursebook, which gave a rule and one clean example per mark and never showed a hard case.
Its limitation matters for anybody being marked. It describes British usage, and the two traditions differ on the serial comma before the final and in a list, and on whether a full stop goes inside or outside a closing quotation mark. It will give you a consistent answer and it cannot tell you which convention your marker wants, which is a question to settle locally and once.
What it looks like when it goes wrong
The recognisable failure is a comma between a subject and its verb, which arrives whenever the subject is long enough to feel like it needs a rest after it. The committee that had been meeting since March, decided to postpone. It reads naturally aloud and it is never correct.
The second is the interruption with one comma instead of two. Opening an aside and never closing it leaves the reader waiting for a boundary that does not come, which is the same fault described from the meaning side in who, which and that, and where the comma goes.
The splice is usually a symptom rather than a cause, since it appears most often where a writer has been cutting long sentences and joined the halves with the nearest mark to hand, which is the subject of length, rhythm, and where a sentence breaks. Both sit under emails, essays and reports that read like a native wrote them.
What this page cannot settle is the other half of what gets marked on a page, which is not punctuation at all and is not a skill either: the same few dozen words are misspelled by almost everybody, and a list is a different kind of problem from a rule: the patterns behind the exceptions, and the words everyone misses.