When is but used as a preposition?

Answer 1

The word "but" is a conjunction, and normally connects clauses. However, one use of "but" seems to be a preposition, as in the sentence:

"No one but me saw the plane."

But is doing the job of the preposition "except" and there is no actual clause. It is actually saying "No one (else) saw the plane, but I saw the plane" where but would be a conjunction.

Answer 2

"But" is used as a preposition in phrases like "nothing but," "anything but," and "everyone but." It is used to indicate exclusion or emphasizing the exception. For example, "I have no choice but to go" means the only option is to go.