When ---当……时候, 通常指某一特定的时间点，主句与从句的动作同时发生。
When I opened the window, I saw him come up.
When --- 正在……的时候，突然…。通常主句是进行时或 be about to 时，在翻译的时候，when 可以译成没想到或突然。
I was walking along the street , when I met him.
When 当从句是进行时，主句是一般时，往往表示不满。
Someone knocked at the door when I was having breakfast.
When the children had gone to bed, she began to prepare her lessons.
While we were inAmerica, we saw him twice.
While ---表示一种不满情绪，意思是这边在干某种重要的事，而另一边在享受等。
We are cleaning the classroom while they are playing the football.
She was doing her homework as she was listening to the music.
As --- 当……时，指一个动作紧接着一个动作发生，从句通常用进行时。
As I was going out, it began to rain.
The moment --- 一……就……=as soon as , immediately,
---Did you remember to give Mary the money you owed her?
---Yes, I gave her the moment I saw her.
He didn’t leave the office until he finished the work.
The passengers should arrive at the airport an hour before the flight
departs.
The customer left the ticket counter after he had a quarrel with the ticket
agent.
Since ---自从……, 通常主句用现在完成时
I have never been there again since I graduated from the university.
It is just a week since we arrived here.
Jack went to school as soon as he got well.
No sooner than --- 一……就……
no sooner… than… 用于句首要求倒装
No sooner had he arrived than he went away again.
Once you see him, you will never forget him.
Every time, each time 每次whenever 每当
Each time he came to town, he would visit our school.
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