Words that are sometimes conjunctions can act as prepositions.
The subordinating conjunctions BEFORE, AFTER and UNTIL can act as prepositions when they are followed by objects rather than dependent clauses.
Remember that a clause has a subject and a verb. A prepositional phrase does not.
PREPOSITION
- Charlie will wait here until sunset.
UNTIL connects the object of the preposition SUNSET with the clause.
NOTE: The entire prepositional phrase is an adverb, not one word.
- It will be some time before summer.
BEFORE connects the object of the preposition SUMMER with the clause.
- After that effort, everyone doubts whether she can win.
AFTER THAT EFFORT is an adverbial phrase and AFTER is a preposition.
CONJUNCTION
- Charlie will wait here until we finish the test.
UNTIL connects the independent clause to the dependent clause.
NOTE: CHARLIE WILL WAIT is connected to WE FINISH.
- It will be some time before the seasons change.
BEFORE connects the IT WILL BE to SEASONS CHANGE.
- After she did so badly, everyone doubted whether she could win.
AFTER connects SHE DID with EVERYONE DOUBTED.
Great summary, thank you.