Commas

Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.

a. Common starter words for introductory clauses that should be followed by a comma include after, although, as, because, if, since, when, while.

While I was eating, the cat scratched at the door.
Because her alarm clock was broken, she was late for class.
If you are ill, you ought to see a doctor.
When the snow stops falling, we'll shovel the driveway.

However, don't put a comma after the main clause when a dependent (subordinate) clause follows it (except for cases of extreme contrast).

1. She was late for class, because her alarm clock was broken. (incorrect)
2. The cat scratched at the door, while I was eating. (incorrect)
3. She was still quite upset, although she had won the Oscar. (correct: extreme contrast)