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.
eg：girls are thus seen as less valuable than boys and art kept at home to do housework while their brothers are sent to school - the prophecy(预言) becomes self- fulfilling, trapping women in a vicious circle (恶性循环) of neglect.
(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.