A dependent clause has a subject and a verb; a phrase does not.
...after lunch (phrase)
...after we ate lunch (clause)
...since Friday (phrase)
...since I left Friday (clause)
...before the session (phrase)
...before the session ended (clause)
The difference matters. For example, at the beginning of a sentence, the phrases above do NOT take a comma; the clauses do.
...After lunch we got down to the business at hand.
...After we ate lunch, we got down to the business at hand.
Happy punctuating!
Margie