What is a preposition?

In, of, on, etc.

It was once explained to me that a preposition is anything a cat can do to a box:

The cat is in the box.
The cat is on top of the box.
Etc.

^I like that.

Parts of speech are confusing. In elementary school, we learn them as strict categories that words belong to, with little gray area, but the actual situation is more complex than that. Some words, like “before”, can be a preposition, adverb, or conjunction depending on context. Parts of speech are something we learn to recognize intuitively, but actually pinning down a concise definition is more difficult. I will attempt it.

A preposition is a word that establishes a semantic relation between words or phrases, that relation often being spatial or temporal. So in the phrase “under the desk”, “under” is a preposition indicating where in space in relation to the desk. In the phrase “after the concert”, “after” is a preposition indicating when in time in relation to the concert.

But that definition is only part of it. You might think of a preposition as a word that allows you to connect one word or phrase to another, and while that connection may be based on location in space or time, it can be many other things too. In the sentence “the book was written by the student”, “by” connects the verb “was written” with “the student”, but what relation is it expressing? It’s not temporal or spatial, it’s expressing a semantic role, i.e. it’s indicating that the student is the agent of the passive verb “was written”. Another example: “we brought gifts for the family”. “For” is a preposition connecting “gifts” with “the family”. In this case it expresses a relation of recipient, i.e. “the family” is the recipient of the gifts.

I hope this it at least somewhat understandable.

A perfectly acceptable thing to end a sentence with.

I always remember “Anything a squirrel can do to a tree”