比较状语从句：
than，as…as…，not so as…，(not) the same as，the+比较级；
eg：Light travels fast than sound.
the+比较级：
eg：The sooner, the better.
（1）当……的时候：while，when，as；
when：强调一件事情正在发生的时候，另外一件事情突然发生；
eg：When I was watching TV, my mother came back.
while：强调两个动作同时进行，一边……一边……；
eg：My father is cooking while my mother is reading newspaper.
as：“随时间推移”
eg：As time went by, the days became longer and longer.
（2）一……就……：as soon as，主将从现；用directly，immediately， instantly连接两个句子；一些名词如the minute，the moment，the instant也可表示；no sooner…than，hardly…when后面的句子需要倒装；
eg：I will write to you the moment/minute I arrived in Paris.
（3）特殊单词：by the time that，every/each time（使用时后面不要再加when）
（4）till&until：
① 肯定形式表示的意思是“做某事直至某时”，动词必须是延续性的；
否定形式表达的意思是“直至某时才做某事”，一般用until，动词为延续性或非延续性都可以；
eg: I slept until midnight.
Wait till I call you.
She didn't arrive until 6 o'clock.
② Until可用于句首，而till通常不用于句首，通常用于句中；
eg: Until you told me, I had heard nothing of what happened.
eg: As you sleep you pass through a sequence of sleep states-light sleep, deep sleep and REM (rapid eye movement) sleep-that repeats approximately every 90 minutes.
