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

PUNCTUATION
Exception
If both the independent clauses in a sentence are brief and simple,
and there is no risk of ambiguity if a comma is not included, it can
be acceptable to omit it. It's never wrong to include it, but sometimes
you may think it overly formal or cluttering. Let the tone and context
of your writing guide your judgment as to whether a comma is really
needed.
This decision is final and there will be no discussion.
The food was okay but the music was lousy.
She begged him to stay so he did.
DON'T USE A COMMA TO SEPARATE INDEPENDENT CLAUSES
THAT ARE NOT LINKED BY A CONJUNCTION
The punctuation in the following sentences is incorrect:
The chairs and coffee tables were worth restoring, the rest was rubbish.
He stealthily parted the curtains, he looked inside.
I think we'll be able to work together, you'd better start being more
punctual though.
You can bake them or fry them or steam them, there are various options.
She could have taken on the task, nonetheless she decided not to.
This type of construction is called a comma splice (splicing means
to unite two things by fastening their ends together), and is an error
because it sends a confusing signal. A comma is intended only for
"light" separations, and sets up an expectation in the reader that
whatever follows it will be closely related to what came before. Accordingly,
the reader isn't anticipating the greater shift in direction that
occurs, and may have to backtrack to make sense of the sentence.
There are three remedies for comma splices: Add a conjunction, break
the clauses into two sentences or use a semicolon instead of a comma.
For a discussion of the last option, turn to page 93.
Exception
If a sentence is short and the style is casual, you can sometimes get
away with a comma splice. It's never technically correct, but you
may sometimes decide that a comma "feels" better than the more
formal semicolon. It's a matter of context.
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