The meanings of these two phrases can be translated in the library. If you only regard the library as a place, use at; If you emphasize in the library, use in. The usage of the two is different.

Phrases are linguistic units that can be collocated at syntactic, semantic and pragmatic levels, and are also called phrases. It is a grammatical unit, bigger than a word, but not a sentence. Simple phrases can be used as syntactic components of complex phrases, and phrases with sentence tone can be used as sentences. A linguistic unit consisting of grammatically collocated words without sentence tone.

A preposition and its object form a prepositional phrase, which is used as an adverbial, predicative, complement, attribute or prepositional object in a sentence. At the same time, the usage of prepositions is also very flexible. The same preposition can express many meanings. Prepositions can be divided into prep. Location preposition, mode preposition, reason preposition, quantity preposition, etc.