时间和原因状语从句
时间状语从句：
引导时间状语从句的连词有：when, while, as, before, after, till, until, since, as soon as, the moment, immediately, every time, next time, the first time, once 等。
        when, while, as表“当...时”的异同 ：        相同点:        当从句的谓语是延续性动词时，三者常                                可互换i.e.        When/While/As we were still laughing, the teacher came in.        不同点:        1) when既表“时间段”又表“时间 点”(=during or                 at the time that),从句中动词既可是终止性的，                        也可是持续性的，还可表“正在这时”i.e. (1). When I arrived there, they were watching                 TV.        (2). When I was young, I liked singing and         dancing.