It centers on modal auxiliaries – words that come before, and change the meaning of, the main verb in English sentences.

这个问题主要涉及的是情态助动词，即出现在英语句子中的主要动词之前并改变其意义的词。

The unstressed words in a sentence are usually auxiliary words, articles and prepositions.

句子中的非重读词通常是助词、冠词和介词。

Speakers of most languages without certain tenses can express the same ideas with auxiliary words, like would or did, or by specifying the time they mean.

大多数语种的说话者都会用" 会" 、" 曾" 这种助动词，或是干脆就具体表明时间。