Because there are so many possible locations, there are quite a few prepositions. Below is the complete list. But is very seldom a preposition. But usually functions as a coordinating conjunction . Understand how to form a prepositional phrase.
Prepositions generally introduce prepositional phrases.
Some prepositions also function as subordinate conjunctions . These prepositions are after , as , before , since , and until . A subordinate conjunction will have both a subject and a verb following it, forming a subordinate clause.