A prepositional phrase is a group of words that begins with a preposition. There is no action in a phrase.
I’ll set the table before dinner.
Let’s take a walk after lunch.
Your dog can stay here until tomorrow.
He has been here since Monday.
In each of these sentences, the first word of the phrase is a preposition: before, after, until, since.
The next word in each phrase is a noun which acts as the object of the preposition.
A clause is also a group of words, but a clause includes a verb and shows action. A subordinate clause sometimes begin with a word that looks like a preposition, but the action in the clause tells us that this is not a prepositional phrase.
Look at these sentences. They begin with the same words as the prepositional phrases above, but these are clauses because they have action.
I’ll set the table before I make dinner.
Let’s take a walk after we eat lunch.
Your dog can stay here until you get back tomorrow.
He has been here since he arrived Monday.
This time, the words before, after, until, and since are not prepositions. They are subordinating conjunctions.
You can tell the difference between a phrase and a clause by looking for action. A clause has action. A phrase has no action.