连词，是一种虚词，不能担任一个句子成分，但可以起连接的作用，即连接词与词或句与句。例如：
We may be leaving today or tomorrow. 我们可能今天或明天走。(连接词与词)
Now I must go or I shall be late for the party. 我现在得走，否则晚会我就要迟到了。(连接句与句)
主要是包括并列连词和从属连词。在句子中，连词起连接词与词、短语与短语及句子和句子的作用。它本身在句子里不单独做句子成分。
并列连词：是用来连接语法地位相同的结构、相同的单词、短语及句子。
并列连词有： and, but, or, nor, so, for yet, however, as well as, both... and, not only... but also, either... or, neither... nor, still,
从属连词：用来引导状语从句。
从属连词有： after, when, before, as, while, since, until, till, if, unless, lest, because, than, that, whether, so that, in order that, as if, as though, although, suppose that, provided, that, as... as, now that, such... that, in case that, on condition that
