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.
While ---在……期间,往往指一段时间.
While I am washing the floor, you can be cleaning the windows.
Before --- 在……之前
Did you work as a stewardess before you come to this company?
After --- 在…… 之后
The plane took off after the air traffic controller gave clearance.
Since ---自从……, 通常主句用现在完成时
It is just a week since we arrived here.
No sooner than --- 一……就……
No sooner had he arrived than he went away again.
Once --- 一但……就……
Once you see him, you will never forget him.
Every time, each time 每次whenever 每当