before引导状语从句用法

before引导的是时间状语，表示在......之前，后面跟完整句子，是时间状语从句。例如三思而行的英语有think twice before you jump。

I had lived here for three years before he came here.在他来这之前，我已生活在这三年了。

I must arrive at cinema before my girlfriend arrives.我必须在我女朋友到达之前到达电影院。

before引导的时间状语从句不能用于否定句。 

例：铃还没响，同学们就走出去了。 The students went out before the bell rang.

before之后只能接时间点，不能在表示“一段时间”的词前作介词，不能说before a month，可说a month ago。

before引导时间状语从句的特殊句式

It will be/+段时间+...“还要过多久才.…”

It was+时间段+...（动作已发生）

It would be +段时间+...

It will not be long before...“不多久就会.…”

It was not long before...“不多久就.…了”

It would be not long before..…“不多久就.…了”

before引导时间状语从句的例句:

It will be four days before they come back.他们要过四天才能回来。

Einstein almost knocked me down before he saw me.爱因斯坦几乎把我撞倒才看到我。

My father had left for Canada just before the letter arrived.我父亲恰好在信到之前去加拿大了。

They had not been married four months before they were divorced.他们结婚还不到四个月就离婚了

连接时间状语从句的连接词有：when， before， after， while， as soon as， until， since......

when， while和as的区别:

when引导的从句的谓语动词可以是延续性的动词，可以是瞬时动词。并且when有时表示“就在那时”。

例如：When she came in， I stopped eating.她进来时，我停止吃饭。(瞬时动词)

While引导的从句的谓语动作必须是延续性的，并强调主句和从句的动作同时发生（或者相对应）。并且while有时还可以表示对比。

例如：While my wife was reading the newspaper， I was watching TV. (was reading是延续性的动词，was reading和was watching同时发生)。

As表示“一边……一边”，as引导的动作是延续性的动作，一般用于主句和从句动作同时发生as也可以强调“一先一后。

例如：We always sing as we walk.我们总是边走边唱。（as表示“一边……一边”）