Do it before it gets worse.(趁着)
Eat it before the food gets cold.(趁着)
Don't let me know what you decide beforeyou think it over.
After you think it over, please let me know what you decide.
After we had finished the work, we went home.(从句用过去完成时，主句用一般过去时)
The fire went on for quite some time _________ it was brought under control.
A. when B. since C. after D. before
【分析】答案 D。最大的陷阱是A when.before 意为“在……之前”，句意是“大火在得到控制之前燃烧了相当一段时间”。又如：
He made a mistake, but then he corrected the situation _________ it got worse.
A. until B. when C. before D. as
Someone called me up in the middle of the night, but they hung up _________ I could answer the phone.
A. as B. since C. until D. before
She is getting better by degrees, but it will be some time _________ she is completely well.
A. that B. since C. when D. before
They sat down opposite each other, but it was some moments _________ they spoke.
A. after B. before C. since D. when
Do it ________ you forget it.
A. after B. before C. until D. unless
The keys: C D D B B
3.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，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时，必须用否定形式;如果主句中的谓语动词是延续性动词时，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.
It was not until the meeting was over that he began to teach me English.
I worked until he came back.我工作到他回来为止。动作结束的时间
I didn't work until he came back.他回来我这才开始工作。动作开始的时间
Please wait until I arrived.
4.由since/before引导的时间状语从句。
一般情况下，从句谓语动词用一般过去时，而主句的谓语动词用现在完成时。但在It is +时间+since从句的句型中，主句多用一般现在时。例如：
I have been in Beijing since you left.
It is four years since my sister lived in Beijing.(动词延续，译为否定句)
It was a long time before I went to sleep again.
It was an hour before(＝until) the police arrived.
She had known something about it long before I told her.
Do it before you forget it.
It wasn’t long before they got married.
5.由as soon as, immediately, directly, instantly, the moment, the second, the instant, the minute, the while 等引导的时间状语从句。这些连词都表示“一……就”。例如：
I will go there directly I have finished my breakfast.
The moment I heard the news, I hastened to the spot.
【注意】hardly(scarcely, rarely)…when / before, no sooner…than相当于as soon as之意。主句用过去完成时，从句用一般过去时。当hardly, scarcely, rarely和no sooner位于句首时，主句应用倒装语序。例如：
He had no sooner arrived home than he was asked to start on another journey.
No sooner had the sun shown itself above the horizon than he got out of bed to commence work.
6.由by the time引导的时间状语从句。
在一般情况下，如果从句的谓语动词用一般过去时，主句的谓语动词用过去完成时;如果主句的谓语动词用一般现在时，主句的谓语动词用将来完成时或将来进行时。例如：
By the time you came back, I had finished this book.
By the time you come here tomorrow, I will have finished this work.
By the time you come back tomorrow, I will be having lessons in class.
By the time you arrived home, I will be studying in this university.
7.由each time\every time\next time/last time/every time/the first time,和whenever/no matter when引导的时间状语从句。例如：
Each time he came to Harbin, he would call on me.
Whenever that man says “To tell the truth”, I suspect that he's about to tell a lie.
No matter when she says ‘to tell the truth’, I suspect that he’s about to tell a lie.
8.由as long as和so long as引导的时间状语从句。这两个连词表示“有多久……就多久”或者“只要”。如 You can go where you like as long as you get back before dark.
I will fight against these conditions as long as there is a breath in my body!
You can stay as long as 10 days.
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