由before和after引导的时间状语从句before的句型:It is/was (not) +时间+before+从句 用了多长时间......注意before引导的从句不再用否定式的谓语,并且当before引导的从句位于主句之后,有时译成“就,才”。还要注意主句和从句之间的时间关系。当主句用将来时,从句总是用现在时;如果before引导的从句谓语用的是过去时,则主句动词多用过去完成时,这样以便体现动作发生的先后。After表示主句动作发生在从句动作之后。主句和从句的动作的时间关系正好与before引导的从句相反。例如:It will be four days before they come back. 他们要过四天才能回来。Einstein almost knocked me down before he saw me.爱因斯坦几乎把我撞倒才看到我。My father had left for Canada just before the letter arrived.我父亲恰好在信到之前去加拿大了。They had not been married four months before they were divorced. 他们结婚还不到四个月就离婚了。After you think it over, please let me know what you decide.你仔细考虑过以后,告诉我你是怎样决定的。After we had finished the work, we went home.完成工作之后,我们回家了。(从句用过去完成时,主句用一般过去时)由till或until引导的时间状语从句till和until一般情况下两者可以互换(till一般不放句首),但是在强调句型中多用until。并且要注意的是:如果主句中的谓语动词是瞬时动词,必须用否定形式;如果主句中的谓语动词是延续性动词,用肯定或否定形式都可以,但表达的意思不同。例如:I didn't go to bed until(till) my father came back.直到我父亲回来我才上床睡觉。It was not until the meeting was over that he began to teach me English.直到散会之后他才开始教我英语。I worked until he came back.我工作到他回来为止。I didn't work until he came back.他回来我这才开始工作。Please wait until I arrived.在我到达之前请等我。