连词
　　并列连词
　　表示意义的引申: and, both…and, not only…but(also), as well as, and …as well, neither…nor
　　表示选择: or, either…or
　　表示转折: but, while, whereas, yet, however/nevertheless(也可认为是副词)
　　表示因果: for, so, therefore, hence
　　从属连词
　　表示时间: when, while, as, after, before, since, until(till), as soon as, once
　　表示原因: because, as, since, now that, seeing that
　　表示条件: if, unless, in case, provided(that), suppose, as long as, on condition (that),
　　表示其他关系: (al)though, than, as/so…as, lest, in order that, so…that