② while 有时可以表示前后两种情况进行对比，作为并列连词 "而，却"

He likes pop music, while I am fond of folk music.

③ While 放在句首引导从句，译为"尽管"

While I think you idea is good, I don't quite agree with everything.

⑶ as的用法

as 表示"当......时候"或"一边......一边"，as 的从句中可使用延续性动词，也可使用非延续性动词。

He sang as he was working.

As time goes by, we have a better understanding of things around us.

As I get older, I get more optimistic.

(4.）until 引导的时间状语从句

Not until 置于句首时，主句的主语和谓语要倒装，表示加强语气。

Not until Mary had heard that sound was she really frightened.

(5 ) as soon as, immediately, directly, once, no sooner....... than, hardly \scarcely......when 引导的时间状语从句，表示"一......就......"

I came immediately I got your message.

Once you lose confidence, you won't be able to do the work well.

Jack had hardly\scarcely seen me when he left the classroom.

※当no sooner, hardly, scarcely 置于句首时，主句要部分倒装.

Hardly\scarcely had we gathered in the wheat when it began to rain.

No sooner had we arrived at the station than the train left.

2． 条件状语从句

① 条件状语从句中，主从句的时态要相呼应，但当条件状语从句表示将来时，从句要用一般现在时，而不用将来时。

If you see him, tell him to come to my office.

Write to me when you have time.

I will go there if (it is) necessary.

② will 用作情态动词表示意愿时，常出现在条件状语从句中。

Come here if you will stay with me.