We followed her up the stairs to a large meeting room.
Put those books up on the top shelf.

It is said that "up" in the first sentence is a preposition, but in the second sentence it is an adverb.

Could you tell me how they differentiate that? (I know adverbs and prepositions in general but I can't understand here).
Hello Gary,

In #1, if you omit "up", you have:

1a. We followed her the stairs.

In 1a, "the stairs" has no relation to anything else in the sentence; the sentence no longer makes sense. So "the stairs" must be the complement of "up"; and "up" is therefore a preposition.

In #2, if you omit "up", you have:

2a. Put those books on the top shelf.

In 2a, the sentence makes sense. So "up" must be an adverb, which modifies the verb "put".

(I don't know if that helps – it's difficult to avoid a certain degree of circularity in these explanations, so let me know if it's still unclear.)

Best wishes,

What about: 'Curl up with a good book.'? The sentence doesn't make sense without 'up', but I would think 'up' is describing 'curl', so should be an adverb.