Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.
*** 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.However, don't put a comma after the main clause when a dependent (subordinate) clause follows it (except for cases of extreme contrast).
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.
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)