Hi Fiona,

In real basic terms, commas are used to present a "pause" in a sentence. The comma usually presents a grammar rule (usually modification or parallelism).

Semicolons are used to present two ideas that could be their own individual sentences, but share a common idea/subject.

GMAT assassins aren't born, they're made,
Rich

Fiona wrote:Can anyone explain the usage of comma and semicolon.

Thanks,
Fiona

I'm happy to help.

Commas are almost always used to separate a subordinate clause from an independent clause. If two independent clauses are separated by a coordinating conjunction that shows a change in direction (e.g. "but", "yet", etc.), this would require a comma. When two independent clauses are separated only by a comma, with no conjunction, this is called a comma-splice, one kind of run-on sentence. See:
https://gmat.magoosh.com/lessons/916-run-on-sentences

Mistakes with commas & conjunctions are common on the GMAT SC. The use of other punctuation is a much rarer topic.

I agree with what Rich says --- sometimes, when we want to show some essential connection between two independent clauses, we might join them with a semicolon.
Seven were accused of the crime; ultimately, only four were convicted.

We must be careful. If the second clause provides some kind of explanation or essential clarification for the first clause, then we might use a colon or a dash. I would say that would be the correct punctuation in melguy's sentence:
They are good friends: they do everything together.
I think the colon, or a dash, works much better in that sentence than does a semicolon.

I don't know that I have every seen colons and dashes, as separators of independent clauses, on the GMAT. The GMAT uses colons for listing examples. The GMAT only rarely has a sentence with a semicolon.

Does all this make sense?
Mike