4种句动作同时发生或从句动作先于主句。
(既可引导时间点又可引导时间段)I was thin when I was a child.The film had been on when w e arrived.2. be about to do … when …be doing …when…be on o ne’s way … when …be on the point of doing …when…had done … when…“ 在那时”“这时”,表示某件事正在发生或刚刚发生，另一动作同时发生The telephone was ringing when I got home.I was about to go to bed when he came back.W e w ere just on the point of calling you up when you came in.3. When 还以引导条件状语从句相当于ifHow can I explain it to you when / if you won’t listen.How can you get good records when you don’t study?When you read it again, the meaning will become clearer to you.4.When还可引导原因状语从句，“既然”It was foolish of you to take a taxi when you could easily walk there in five minutes.3种1. 必须引导持续性动作，强调在一段时间内，主句和从句动作同时发生。
My mother was cooking while I was doing my homework.Please don’t talk so loud while others are working.2. “然而” 表示轻微转折, 两者对比。
I like watching TV while he likes reading.3.引导让步状语从句“ 虽然，尽管”While I admit his good points, I can see his shortcomings.1 As I left the house , I forgot the key.我离开家时，忘记了带钥匙。
As 引导时间状语从句，强调动作并行发生，不指先后。
2. As I get older, I get more optimistic.随着年龄的增长，我变得更加乐观“随着” 表示时间的推移。
3. He hurried home, looking behind as he w ent.他匆匆忙忙回家，边走边往后看。
“一边…一边…”4 As he was going out, it began to rain.当他出去时开始下雨了强调两个动作紧接着发生。
5. As a boy (When he was a boy), he was hopeless at English.当他是孩子时，就对英语失去了希望。
As有时引导一个名词，相当于时间状语从句。
As I get older, I get more optimistic. （时间）Y oung as he is , he knows a lot. （让步）As he wasn’t ready in time, w e w ent without him. （原因）Do as I told you. / do as you like. （方式）He is as old as me.(比较)I’m happy as long a s you are happy. （条件）1. (04北京春) W e were swimming in the lake ______ suddenly the storm started.A. whenB. whileC. untilD. before2. (02上海) He was about to tell me the secret ______ someone patted him on the shoulder.A. asB. untilC. whileD. when3. (05上海) He transplanted the little tree to the garden ______ it was the best time for it.A. whereB. whenC. thatD. until4. (05福建) －Did Jack come back early last night?－Y es. It was not yet eight o’clo ck ____ he arrived home.A. Bef oreB. whenC. thatD. until（注：句型：It is/was + 点时间+when从句）5. (06天津) The cost of living in Glasgow is among the low est inBritain, ______ the quality of life is probably one of the highest.A. sinceB. whenC. asD. while6. (04年江苏) ______ I accept that he is not perfect, I do actually like the person.A. WhileB. SinceC. Bef oreD. Unless7. (06辽宁) He was about halfway through his meal ______ a familiar voice came to his ears.A. whyB. whereC. whenD. whileBefore 的用法4种1.在…之前2.It will (not) be +时间段＋before＋从句“ 过了多久就、才… ” ; “过不了多久就/才…”3. “还没来得及，就…”4. “趁……”1.Before they got to the bus stop, the bus had gone. 在..前2.It will be 5 years before he returns to his motherland. 过多久才…3.W e waited a long time before he came out.过了多久就、才…4. It won’t be long before w e meet again.过不了多久就5. He died before he wrote a will. 还没来得及6. Catch him before he escapes. 还没来得及7. Please write it down before you forget it.趁着1. (03年北京) He made a mistake, but then he corrected the situation ______ it gotworse.A. untilB. whenC. beforeD. as2. (04福建) Scientists say it may be five or six years______ it is possible to test thismedicine on human patients.A. sinceB. afterC. beforeD. when3. (06四川)－Why didn’t you tell him about the meeting?－He rushed out of the room____I could say a word.A. beforeB. untilC. whenD. aftertill, until和not…until：1.until/till引导时间状语从句用于肯定句时，主句的动词是延续性动词，表示动作或状态一直持续到until/till所表示的时间，意为“某动作一直延续到某时间点才停止”。
如：W e waited until/till he came.2．用于否定句时，主句谓语动词是非延续性动词，从句为肯定，意为“某动作直到某时间才开始”。
如：He won’t go to bed till/until she returns.3．till不可以置于句首，而until可以。
如：Until you told me I had no idea of it.4．not…until句型中的强调和倒装说法：It was not until you told me that I had any idea of it.Not until you told me did I have any idea of it.1 (03北京春) －W as his father very strict with him when he was at school?－Y es. He had never praised him ______ he became one of the top students in his grade.A. afterB. unlessC. untilD. when2. (03上海) A good storyteller must be able to hold listeners’ curiosity ______ he reaches the end of the story.A. whenB. unlessC. afterD. until3. It was not ______ she took off her dark glasses______ I realized she was a famous film star.A. when; thatB. until; thatC. until; whenD. when; thenMr li has been here since 1998.Mr li has been here since he came back.It’s/ha s been tw o years since w e arrived here.我们到这儿两年了。
It is three years since he lived here. (延续）他不在这儿住三年了。