when作并列连词构成并列句,意为and at that time“正在那时”,and just then“那时”或and then“突然”等意思.when前可以用逗号,也可以省略.同时,when可与suddenly连用以加强语气.这种结构常常用来表示发生的事情带有意外的性质.其句型有三类：
1.sb.was doing sth.when （suddenly）...表示“某件事情正在进行时,另一件事也发生了”.
We were swimming in the lake when suddenly the storm started.我们正在湖里游泳,这时突然下起了暴风雨.
2.sb.was （just） about to do...when （suddenly）...表示某人刚要开始做某事,（突然）另一件事发生了.
he was about to tell me the secret when someone patted me on the shoulder.他刚想把这秘密告诉我,这时某个人拍了拍我的肩膀.
while 用作连词时,有以下几种含义：
1.意为“当……时候,和……同时”,引导时间状语从句.例如：
While I was watching TV,the bell rang.
我看电视时,铃响了.
Meimei watched TV while she ate her supper.
梅梅边吃晚饭边看电视.
2.意为“虽然,尽管”,引导让步状语从句.例如：
While it was late,he went on working.
虽然很晚了,但他还在继续工作.
While he is in poor health,he works hard.
虽然他身体不好,但他还是努力工作.
3.连接两个并列句子,表示“对比”关系.例如：
He went out for a walk,while I stayed at home.
他出去散步了,而我却呆在家里.
I like singing while she likes dancing.
我喜欢唱歌,而她喜欢跳舞.
You like sports,while I prefer music.
你喜欢体育,而我更喜欢音乐.
after的用法：
after引导的时间状语从句如果具有将来意义,往往要用一般现在时来表示(有时也用现在完成时),而不能直接用一般将来时.如：
误：I’ll tell him after you wil leave.
正：I’ll tell him after you leave (或have left).你走了之后我再告诉他.
(2) 选择一般现时或现在完成时(表示将来意义)的基本原则是：如果强调从句 动作在主句动作之前完成,则用现在完成时；如果强调主从句动作 同时发 生,或几乎同时发生(尤其是当从句的谓语动词是get,arrive,see,hear,leave,return等表示迅速完成的动作的动词时),多用一般现在时(有时 两种时态都可以).如；
I’ll leave when he comes back.他回来后我就离开.
I will give my opinion when I have read the book through.我把书看完了再提意见.
We’ll leave as soon as it stops (或has stopped) raining.雨停我们就动身.
2.一般说来,如果在过去某一时间内先后发生了两个动作,那么先发生的动作用过去完成时,后发生的动作用一般过去时.但是若用连词after 来连接这两个动作,由于after本身已说明了两个动作的先后关系,所以两者均可用一般过去时.如：
After he (had) closed the door,he left the house.关了门之后,他就离开了家.
3.有时以连词after引导的时间状语从句可以换成一个以介词after引导介词短语或以连词after引导的分词短语,而意义不变.如：
He went to school after he had breakfast (=after breakfast).吃了早饭之后,他就去上学了.
After l had finished (=After having finished / After finishing / Having我干完活后就去睡觉了.
4.after还可用作副词,意为“后来”或“(一段时间)以后”,通常要放在另一时间副词之后.如：
We arrived soon (或just / shortly) after.我们不久就到了.
He returned twenty years after.20年后他回来了.
before的用法
1.表示“在…..之前就…..”.
Mary had finished her homework before her mother returned.
2.表示“过了多久后才……..,动作进行到什么程度才……”
The man almost knocked me down before he saw me .
They walked about 30 miles to the west before they saw a village .
They worked day and night about three days before everything returned to normal .
3.表示“刚…就…”
I had hardly sat down before the bell rang.
4.表示“不知不觉就…..； 还没弄清就……”
Time passed quickly and three months went by before Aqiao knew it.
5.用于句型“it was +时间段+ before …” 表示 “过了多久才…”
It was three days before he came back..
6.用于句型“ it was not long +时间段+ before …” 意为“不久就….”
It was not long before he sensed the danger of the position .
7.用于句型“it will be(not )+时间段+ before….”表示“要过多久/ 不久…才…”
It will be half a year before you graduate from this school .
It will be not long before you find you are wrong .
