一．状语从句：
1.定义：用作状语的句子，相当于副词的用法；
2.分类：时间状语从句；原因状语从句；条件状语从家；结果状语从句；让步状语从句等。
二．时间状语从句：
1. when, while和as都意为“当……的时候”；表示前后动作基本同时发生，引导时间状语从句；
(1)when引导的从句谓语动词可以为延续性动词和非延续性动词；主句和从句的主语不一样；
(2)while引导的从句谓语动词必须为延续性动词，通常用于进行时；主句和从句的主语不一样；
(3)as引导的从句谓语动词通常为延续性动词，通常用于进行时；意为“当……的时候”时，主句和从句的主语通常不一样；当表示“一边……一边……”时，主句和从句的主语一致。
e.g. I am doing my homework when Tom comes back.
I was doing my homework while/when /as my mother was doing breakfast yesterday morning.
Tom is reading a book as he is listening to music now.
When/While (she was) walking in the park, she saw an old friend.
When it is seen from the hill, the park looks very beautiful.
When it is heated, ice will be changed into water.
(4) be about to do意为“刚要……；正要……”，表示即将发生的动作。不与具体时间连用，常与when搭配，此时的when意为“突然，正在这时”，引导时间状语从句。
e.g. The concert is about to begin.
She was about to leave when some guests came.
(5) while(然而，但是)和but都表转折时的区分：but只表示前后状况相反或不一样，而while表转折时有鲜明对比在里面；
e.g. Some people are rich while/but others are poor.
Tom likes playing basketball, but /while Jack likes playing football.
2. since意为“自从”，引导的时间状语从句，从句用一般过去时，主句用现在完成时且主句谓语动词通常为延续性动词；
e.g.Tom has lived in Kunming since he came here.
My parents have been away from my hometown since I went to the university.
注意：Since可意为“既然”，相当于now that;
e.g. Since she said like this, I think we should believe her.
3. before和after作连词时的用法：
(1) before 意为“在……之前”，引导时间状语从句时，若从句用一般现在时，那主句用现在完成时；若从句用一般过去式，那主句用过去完成时。
(2) after”在……之后”，为before的反义词，也可以引导时间状语从句；
e.g. Tom had lived in Kunming before he began to learn English.
I have lived here before Mary comes here.
She has studied French after she made the decision.
After he had finished the work, he went to see his teacher.
4. as soon as意为“一……就……”, 若主句用将来时，从句则用一般现在时；若从句用一般现在时，主句不一 定用将来时;
e.g. I will call you as soon as I arrive in Beijing.
Tom will come to visit you as soon as he finishes his homework.
5. till, not…until意为“直到……才…”,引导时间状语从句；
e.g. Tom didn’t come back home until I finished my homework.
Mary doesn’t speak English until she is eight years old.
6. directly，immediately，instantly等副词引导的时间状语从句，意为“一……就……”;
e.g. Instantly my mother came back yesterday, she went to cook dinner.
Directly Tom did the thing, he felt sorry.
7. the moment(一……就……)，the minute(一……就……)，the instant(一……就……)，every time(每次)，each time(每次)等短语引导的时间状语从句；
e.g. The instant the result came out, our English teacher told us.
Each time Tom met his math teacher, he felt a little nervous.
8. hardly/scarcely/barely…when/before…，no sooner…than…引导的时间状语从句，意为“一……就……”；注意前半句要用过去完成时，后半句要用一般过去时；
e.g. Tom had hardly/scarcely/barely seen me when/before he left the classroom.
I had no sooner called my mother than I got home.
9.注意：在使用时间状语从句时，如果主句为一般将来时，那么从句要用一般现在时。
e.g. Students will go home after they finish their exam.
I will call you as soon as I arrive in Beijing.
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