状语从句在句中作状语,修饰主句中的动词、形容词或副词等。状语从句放在主句之前时,常用逗号分开;放在主句之后,一般不用逗号。状语从句按其意义和作用可分为时间、条件、原因、让步、目的、结果、方式、比较、地点等9种。
(1)时间状语从句:
①时间状语从句常用连词有when,as,while,before,af-ter,since,till(until),as soon as,hardly...when...,no sooner...than,the moment等;
②时间状语从句一般不用将来时,因此,主句若为将来时,时间状语从句要用一般现在时。例如:
I'll go on with the work when I come back tomorrow.
③连词when,while,as表示"当……时候",但有区别。as多用在口语中,强调"同一时间"或"一前一后",有时还有"随着"的含义。如:
As he was eating his breakfast,he heard the doorbell ring.
(强调同一时间)
As I was going out,it began to rain.
(强调两个动作紧接着先后发生)
As spring warms the good earth,all flowers begin to bloom.
(as有"随着"的含义)
when强调"特定时间"。例如:
When the clock struck twelve,all the lights went out.
When he was eating his breakfast,he heard the door bell ring.
while表示的时间是一段,而不是一点。while有时还有对比的含义,意为"而"。例如:
While he was eating his breakfast,he heard the doorbell ring.
(强调吃饭的过程)
While I was reading,he was playing.
(有对比的含义)
④till(until)表示"一直到……"时,主句和从句都用肯定式;表示"直到……才……"时,主句用否定式,从句用肯定式。例如:
He worked until it was dark.
他一直工作到天黑。
He did not get up till his mother came in.
母亲进来他才起床。
⑤as soon as和the moment引导的从句表示"一……就……";用no sooner...than和hard-ly...when引导的从句表示"刚……就……"。主句中的动词一般用过去完成时,从句用过去时。主句一般倒装,把助动词had提到前面。例如:
As soon as I finish the work,I'll go to see you.Hardly had I entered the room when I heard a loud noise.
