Here's a breakdown:
* "Once" as an adverb:
* No comma needed: If "once" means "one time," it typically acts as an adverb and doesn't require a comma.
* Example: "Once upon a time, there was a princess."
* "Once" introducing a dependent clause:
* Comma needed: If "once" introduces a dependent clause, you need a comma.
* Example: "Once the sun sets, the stars will appear."
* "Once" introducing a contrasting idea:
* Comma needed: If "once" introduces a contrasting idea, a comma helps to separate the two thoughts.
* Example: "I used to love pizza, once I became vegetarian I stopped eating it."
In summary:
* Use a comma before "once" if it introduces a dependent clause or a contrasting idea.
* Don't use a comma before "once" if it's acting as an adverb meaning "one time."
If you're unsure, read the sentence aloud. If you pause before "once," you likely need a comma.