In terms of category of words, it can function as notional word, such as verb, pronoun, conjunction, as well as functional word, such as auxilary word, emotional word, etc.
在词类上，它可以是实词，做动词、代词、连词等； 也可以是虚词，做助词、语气词等。
2
But "is a very commonly used word. It can be used as a conjunction, a preposition, an adverb and a relative pronoun in sentences."