1. Before she could move, she heard a great noise, which grew to a terrible roar.
还没来得及动，她就听见一声巨响，接着就是可怕的隆隆轰鸣声。
before用作连词，其后接从句时，表示“在…之前”。在不同的语境中，它往往有比较灵活的用法，如“趁…”，“还没来得及就…”(表时间快)，“过了…才”(表时间慢)等。
[例] It was a week before he could tell his experience.
过了一周之后他才能讲述他的经历。
It will be five months before we meet again.
还要过五个月之后我们才能再见面。
I slipped out before the meeting ended.
还没等会议结束我就溜了出来。
She ran away before I could explain to her.
我还没来得及向她解释她就跑了。
He asked a second question before I could answer the first question.
我还没来得及回答第一个问题，他又问了一个问题。
The fire lasted about four hours before the firefighters could control it.
大火持续了大约四个小时，消防人员才得以控制。
I must write it down before I forget it.
趁着我还没忘，我得把它记下来。
[典题示例]
(1) —How long do you think it will be ______ China sends a manned spaceship to the moon?
—Perhaps two or three years.
A. when B. until C. that D. before
答案：D
解析：题意为“要过多久之后中国才可以发射载人飞船到月球上去”。before意为“在…之前”，此处可译为“过了多久才…”。
(2) —Why didn’t you tell him about the meeting?
—He rushed out of the room ______ I could say a word.
A. before B. until C. when D. after
答案：A
解析：before此处言其动作快，即：还没等我来得及说上一句话，他就冲了出去。
2. However, before she could think twice, the water was upon her.
然而，转念之间，瘩水已近在眼前。
句中的think twice是惯用语，意为“细细地想一想，再想一想(to think very carefully about sth.)”.
[例] I should think twice before accepting that offer.
是否接受这种帮助，我还得细细地想一想。
Think twice before you act.
你可要三思而后行啊。
You must think twice before you decide on the plan.
在对这项计划做出抉择前你要三思。
句中的be upon sb.意为“向某人逼近；临近”。
[例] The enemy was upon us.
敌人已逼近我们。
The Spring festival will soon be upon us.
春节快到了。
3. Tree after tree went down, cut down by the water, which must have been three metres deep.
一棵接一棵的树倒下了，是被洪水冲倒的，那水一定有三米深。
1) tree after tree相当于one tree after another，意为“一棵接一棵的树”。
n. + after + n. 是一习惯表达，意思是“一个又一个的…”，此时名词用单数，且不能加冠词。多用来表“重复性”。
[联想] year after year年复一年
month after month一月又一份
week after week一周又一周
day after day日复一日
shop after shop一家又一家商店
village after village一村又一村
这种短语也可用“one + 名词 + after + another”来表示。例：
one week after another, one shop after another, one victory after another等。
[例] Day after day passed by without a line from him.
日子一天天过去了，但没收到他一封信。
She sent him letter after letter to explain the delay.
她给他写了一封又一封信，向他解释推迟的原因。
In factory after factory we were shown machines produced by the workers themselves.
在一家又一家工厂里，我们看到了工人们自己生产的机器。
2) must have done用于对已发生的情况的肯定推测，意为“一定…”。
[例] I didn’t hear the phone. I must have been asleep.
我没有听见电话，我一定是睡着了。
You must have seen the play last week.
上周你一定看了这部戏。
[典题示例]
(1) —The biologist woman stayed in Africa studying wild animals for 13 years before she returned.
—Oh, dear! She ______ a lot of difficulties!
A. may go through B. might go through
C. ought to have gone through D. must have gone through
答案：D
解析：此处表对已发生之事的推测。即：她一家经历了许多困难。
(2) —Guess what! I have got A for my term paper.
—Great! You ______ read widely and put a lot of work into it.
A. must B. should C. must have D. should have
答案：C
解析：情态动词 + 动词原形，是谈现在或将来之事；情态动词 + have done结构是用来谈已发生之事。must + have done表对已发生之事的推测；should + have done表“过去应该做（但未做）之事”。
4. The monkeys look sweet, but they can by very naughty.
这些猴子很可爱，但它们有时候也很顽皮。
句中的can be very naughty意为“有时候会很顽皮”。
can这里不表推测，而是表示“客观可能性”，指一时的情况，即：有时候会…。
[例] Our house is on the top of the hill, and in winter the winds can be pretty cold.
我的家在一个小山顶上，冬天的风有时会很冷。
He is a bad-tempered fellow, but he can be quite charming when he is with his children.
他的脾气很坏，但当他与孩子们在一起时，他有时很可爱。
[典题示例]
(1) The World Wide Web is sometimes jokingly called the world wide wait because it ______ be very
slow.
A. should B. must C. will D. can
答案：D
解析：此处can表“有时候会…”，不是表推测，而是一种客观上存在的事实。
(2) Some aspects of a pilot’s job ______ be boring, and pilots often ______ work at inconvenient
hours.
A. can; have to B. may; can C. have to; may D. ought to; must
答案：A
解析：第一空同上；第二空用have to表示“不得不”之意，即：无奈之意。
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