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。希望通过本文的学习，考生能对四级语法有更全面的掌握!

　　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.