Grammatically Correct: The writer's essential guide to punctuation ...

Comma(,)
The comma is by far the most-used punctuation mark, typically
outnumbering all the others put together. Its basic role is to function
as an interrupter, separating a sentence into distinct units.
Most of the comma's numerous functions fall into the following
categories:
• Separating the main elements of a sentence from each other
• Setting off a parenthetical element from the rest of the sentence
• Separating elements in a series
• Setting off dialogue or quotations
• Indicating omitted words
Note: Before beginning, be sure that you have reviewed "Basic
Sentence Structure" on page 55.
A VOIDING COMMAS WITHIN CLAUSES
Using the comma properly means knowing both where to put itand
where not to. Breaking up a string of words that ought to be
treated as an indivisible unit will send just as confusing a signal to
the reader as letting discrete units run into each other. This section
reviews the main scenario of an "indivisible unit"; other scenarios
where commas are not appropriate are described farther on.
DON'T SEPARATE ANY OF THE MAIN PARTS OF A CLAUSE
A clause focuses on a single idea or on closely related ideas, and
therefore should not be broken up. In general, you should not put
commas between the subject and the predicate, between the parts of
a compound subject or between the parts of a compound predicate.
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