1) 时间状语从句

　　时间状语从句主要由以下连词引导：when, whenever, as, while, since, until, till, before, after, as soon as, once, hardly (scarcely) …when, no sooner …than。

　　I will discuss the matter with you when we meet tomorrow.

　　时间状语从句还可由某些可充当连词的名词(the moment, the instant, every time,等)和副词(instantly, directly等)引导：

　　I will tell you the news the instant I know.

　　Note:

　　①before除了可表示“在…之前”外，还可用来强调主句所表达的时间、距离很长或花费的力量相比较大，可译为“…才”。(“not …before”可用来强调主句所表达的时间、距离很短或花费的力量相比较小，可译为“…就”)。

　　It was a long time before I got to sleep again.

　　②如果位于when引导的分句之前的主句使用过去进行时、过去完成时或“was/were about to, was/were on the point of”等结构时，when表示突然发生某事，可译为“正在…突然”或“刚…就”等。这时，不能够用as或while来代替when。

　　I was walking on the street when I ran into my old friend John.

　　2) 条件状语从句

　　条件状语从句主要由以下连词引导：if, unless, providing/provided (that), suppose/supposing (that), on condition (that), as/so long as, in case等。

　　So long as you work hard, you will surely achieve good results in examinations.

　　3) 让步状语从句

　　让步状语从句主要由以下连词引导：though, although, even if, even though, whatever, however, whoever, wherever, no matter how/ what/ where/ when等。以下介绍几种较特殊的让步状语从句。

　　(1)while引导的让步状语从句多放在句首，while相当于although，表示“尽管”，“虽然”。

　　While I admit I did it, I didn’t intend to.

　　(2)短语“even now/then/so”相当于“though it is/was true”，表示“尽管如此”，“尽管这样”。

　　The fire was out, but even so, the smell of smoke was strong.

　　(3) as引导的让步状语从句要倒装。

　　Child as he was, he could speak four languages.

　　4) 比较状语从句

　　比较状语从句主要由以下连词引导：(not) as …as, (not) the same as, (not) such …as, not so …as, than, the + 比较级…, the +比较级…等。

　　We have had a lot more rainfall this year than we had last year.