大学英语四级考试涉及的语法知识点多、涉及面宽。对不少考生来说都是一个难题，希望通过本节的学习，考生能对四级语法有更全面的掌握!

5. 比较状语从句：

than，as…as…，not so as…，(not) the same as，the 比较级;

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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.

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