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.

 四级答案