A preposition is a word or group of words that shows direction, location or time.

介词是表示方向、地点或时间的一个词或一组词。

Prepositions - words or groups of words that are used with nouns, noun phrases, or pronouns to show direction, location, and so on - are a difficult subject.

介词——与名词、名词短语或代词一起使用的表示方向、位置等的单词或词组——学起来不轻松。