The difference between a dependent clause and a prepositional phrase is that the clause has a subject and a verb.
...We will meet after dinner. (phrase)
...We will meet after we eat. (clause)
...I have not seen him since Friday. (phrase)
...I have not seen him since we met in Barcelona. (clause)
The difference between a dependent clause and an independent clause (a sentence) is that a dependent clause has a word out in front of it that keeps it from standing alone.
...He left early. (independent clause)
...because he left early (dependent clause)
...whether he left early (dependent clause)
...if he left early (dependent clause)
So when you are looking for dependent clauses, you are looking for one of these intro words and a subject and a verb.
To be continued...
Happy punctuating!
Margie