We know that a preposition is a word that shows position. A preposition always begins a prepositional phrase.
A subordinating conjunction is a word that begins a subordinate clause.
The subordinating conjunction connects a subordinate clause to an independent clause.
These words act as both preposition and subordinating conjunction: before, after, until, since
A preposition begins a prepositional phrase. The prepositional phrase shows the position of something.
There is no action in a phrase.
Look at these words being used in a prepositional phrase:
I want to talk to you before dinner.
Sam has tennis practice after school.
It’s supposed to be warm until next week.
It’s been raining since Monday.
Each phrase in darkened letters shows the position of something, but there is no action.
Now look at these same words being used as subordinate clause words (subordinating conjunctions).
I want to talk to you before you eat dinner.
Sam has tennis practice after he finishes class.
It’s supposed to be warm until that cold front arrives.
It’s been raining since we came home.
Each clause in darkened letters begins with the same words as first set of sentences. The difference is that these clauses have action. Can you find the action word in each clause? That action word makes the clause different from a phrase.
When the words before, after, until, since are part of a phrase with no action, they are prepositions. When the words before, after, until, since begin a clause that has action (and a verb) inside, they are subordinating conjunctions.