Is in an adjective or an adverb?

Answer 1

The word "in" is usually a preposition (within, inside), e.g. in town.

Without an object, it is an adverb (come in, settled in).

The only common uses as adjective are to mean modern or fashionable (e.g. the in crowd) or exclusive (an inside or in joke, an in reference).

Answer 2

The word "in" can be an adjective (the doctor is in), an adverb (he came in), or a preposition (in the house). It can arguably be a noun (and is a slang term for a connection to an operation).