How are subordinating conjunctions different from prepositions?
Some subordinating conjunctions (especially those expressing time) can also act as prepositions. As prepositions, they introduce a prepositional phrase without a subject and verb: • After dinner, we’ll leave for the party. (prepositional phrase with no subject and verb) As conjunctions, they introduce a clause containing a subject and verb: • After we eat dinner, we’ll leave for the party.