Commas: where NOT to use them
One place where commas are NOT found (or at least they shouldn't be found) is between a subject and its predicate, that is, between a noun and a verb. What I mean is that commas should not come between a subject and its verb unless there are two of them, and they are there to set off "sentence interrupters." (See below.)
Apparently I am not the only English instructor to find that students want to put commas here. Christopher Altman, an Assistant professor at a college in New York, has written an article mentioning that this mistake is quite common, and explaining that this is wrong.
[By the way, did you notice the notice the commas in the "sentence interrupters," as Altman calls them, and the appositive construction above?]
Another error is the comma splice. A fairly common problem among novice writers, this is the mistake of attempting to join two independent clauses together with nothing more than a comma. When only a comma is placed between two clauses, the result is a run-on sentence.
Altman points out another error which writers make quite frequently. This is the mistake of putting a comma after "which," when actually the comma should come before "which." The following sentence illustrates.
The student forgot to put a comma before the non-restrictive clause, which was a mistake.