Prepositions are words that create a relationship in space or time between other words. Common prepositions include above, by, over, before, after, in, around, and since.
The coat is in the closet.
Sarah arrived before Tim.
In the first sentence, the preposition “in” is used to describe the coat’s location in relationship to the closet. In the second sentence, the preposition “before” is used to describe the relationship in time between Sarah and Tom’s arrivals.
Grammar purists say that you should never end a sentence with a preposition. There are many circumstances where this is true; however, there are cases when it is perfectly acceptable.
You should never end a sentence with an extraneous preposition.
Where are you at?
Where did he go to?
The words “at” and “to” are unnecessary; therefore, you should remove them.
On the other hand, if removing the preposition or restructuring the sentence would cause confusion or convoluted written or verbal gymnastics, it is perfectly acceptable to end your sentence with a preposition.
What did she sit on?
On what did she sit?
The first sentence does end with a preposition; however, it reads and sounds much more natural than the second sentence.
It is also acceptable to end a sentence with a preposition when using a phrasal verb. Phrasal verbs contain multiple words, one of which is always a preposition. “Leave off,” “go ahead,” and “run over” are all considered phrasal verbs.
I told Sally to go ahead.
I wish Bill would cheer up.