助动词是一种可以改变主要动词意义或时态的一组动词，在印欧语系语言中非常常见。例如英语中的have、can、will、must等。这些助动词不能自己单独使用，必须要与另外一个动词配合使用才有意义。又助动词在句子中有其固定的出现位置，例如：在直述句中，助动词通常会出现在主词之后，但却在动词之前： "You must go home tonight." 这个句子中，you 是主词，must 是助动词，go 是动词。另外与助动词连用的动词，必须以动词原型或分词形式呈现。

下面是助动词为现在时和过去时比较

An auxiliary verb (abbreviated aux) is a verb that adds functional or grammatical meaning to the clause in which it appears—for example, to express tense, aspect, modality, voice, emphasis, etc. Auxiliary verbs usually accompany a main verb. The main verb provides the main semantic content of the clause. An example is the verb have in the sentence I have finished my dinner. Here, the main verb is finish, and the auxiliary have helps to express the perfect aspect. Some sentences contain a chain of two or more auxiliary verbs. Auxiliary verbs are also called helping verbs, helper verbs, or (verbal) auxiliaries.