It is neither. Alas is an interjection (utterance). It can also be considered a sentence connector, e.g. There were, alas, no survivors.
It is an adverb. It cannot be a preposition. It may be considered a noun in some uses.
On can be an adverb a preposition or an adjective. adverb - Turn the lights on. adjective - The switch is in the on position. preposition - I'll see you on Sunday.
No. Almost is an adverb. It is not used as a preposition.
It can be either. If it takes a noun as an object, it is a preposition. It if stands alone or with other adverbs, it is an adverb. It can, rarely, be an adjective.
It can be either. If it has a noun as its object, it is a preposition. Standing alone, it is an adverb (and possibly an adjective). It can also be a conjunction.
No, it is not a preposition. Tall is an adjective and possibly an adverb.
It is never an adverb. It is always a preposition. The word "within" can be an adverb or a preposition, and the word "forthwith" (immediately) is an adverb.
It is never an adverb. It is always a preposition. The word "within" can be an adverb or a preposition, and the word "forthwith" (immediately) is an adverb.
preposition
Adverb: I had a hat on. Preposition: I didn't have any money on me.
In this adverb form of "with" meaning "accompanying," along is an adverb and with is a preposition.
No, "throughout" is not a preposition. It is an adverb or a prepositional phrase that functions as an adverb.
No, "together" is not a preposition. It is an adverb that is used to indicate two or more people or things being in one place or gathered as a group.
No, "twice" is not a preposition. It is an adverb that indicates something is occurring two times.
It can be either. It can be a preposition, or it can be an adverb (walking about), or even possibly an adjective (up and about).
No, it is not a preposition. Usually is an adverb.
"some" can function as a determiner, pronoun, or adverb, but it is not a preposition.