可能选出一把钥匙说:
This may/might be the key.
可能就是这一把钥匙。(也许这是那把钥匙。)
但一连试了两把钥匙都没打开锁,就会拿起第三把钥匙说:
This must be the key.
一定是这把钥匙。(不存在别的选择了。)
(b)—I wonder why Tom hasn't answered my letter.
—He may/might be ill.
—我不知道汤姆为什么没给我回信。
—他可能生病了。(但也存在其他可能性:说不定他出门了,或者事情太忙无暇写信。)
但假设比尔从来没有人来访问他。如果急救车停在他门口,邻居们会说Bill must be ill(比尔一定得了病了)。这是对于急救车来到他家一事可能做出的唯一解释。
(c)同样,在谈到过去的动作时:
He may have come by train.
他也许是乘火车来的。(但也存在别的可能:他可能是乘出租汽车或公共汽车来的。)
但He must have come by taxi(他一定是乘出租汽车来的)意指他没有其他选择,不存在用其他办法进行这种旅行的可能。
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[Amber demo]
158 have/had表示推断
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这是美国人的用法,但有时英国人也这么用。
have/had主要是用在与to be连用时:
—There's a tall gred bird fishing in the river.
—It has to be/must be a heron.
—河里有一个大个的灰鸟在捕鱼。
—那一定是一只苍鹭。
had+to be可表示说话人对过去可能发生的事认为是肯定无疑的:
There was a knock on the door.It had to be Tom.
有人敲门。肯定是汤姆。(他肯定是汤姆。)
had+to be也可用来代替 must+完成式:
—I wonder who took the money.
—It had to be Tom./It must have been Tom.He's the only one who wasthere.
—我不知道谁把这钱拿去了。
—肯定是汤姆。当时只有他在那里。
但为了避免混淆,建议学生们还是尽量用must的各种形式。
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[Amber demo]
159 can't和couldn't表示否定的推断
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A 对现在事件的否定推断,可以用can't或couldn't加上动词be的现在式或者可以用can't/couldn't加上动词的进行式表示:
Child:Can I have some sweets?I'm hungry.
孩子:我可以吃些糖果吗?我饿了。
Mother:You can't/couldn't be hungry.You've just had dinner.
母亲:你不会饿的。你刚吃过饭。
Ann(looking through binoculars):An aeroplane is pulling up people fromthe boat!
安(透过望远镜):有一架飞机正把人们从小船吊到飞机上!
Tom:It can't/couldn't be an aeroplane.It must be a helicopter.
汤姆:不可能是一架飞机。一定是一架直升机。
—He says he's still reading The Turn of the Screw.
—He can't/couldn't still be reading it.I lent it to him ages ago and it's quite a short book.
—他说他还在读《螺丝在拧紧》。
—他不可能还在读那本书。我很久以前就把书借给他了,而且书也很薄。
B 对过去事件的否定推断,可以由can't/couldn't+动词的完成式或完成进行式表示:
—A man answered the phone.I suppose it was her husband.
—It can't/couldn't have been her husband.He's been deaf for ages.
—是一个男人接的电话。我想大概是她丈夫。
—不可能是她丈夫。他耳聋了许多年了。
—I took a Circle Line train to St Paul's.
—You can't/couldn't have taken the Circle Line.It doesn't go through St Paul's.You must have been on the Central Line.
—我乘地铁环线去了圣保罗大教堂。
—你不可能乘环形地铁。它不经过圣保罗大教堂。你一定是乘的中央线地铁。
如所表示的推断是过去做出来的或是由动词过去式引导的,就必须用couldn't:
She said I couldn't have come on the Cricle Line.
她说我不可能是乘地铁环线来的。
He said it couldn't be an areoplane.
他说那不可能是飞机。
否则can't或couldn't都可以使用。
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[Amber demo]
160 will和should表示假设
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A will用于表示对现在或过去行为的假设
will可与现在式(只限于表示未经预先考虑或计划的行为)或进行式或完成式连用:
Ring his home number.He'll be at home now.
拨他住宅号码。他现在会在家。(我肯定他在家。)
He'll be expecting a call from you.
他该在等你的电话。(我肯定他在等电话。)
He'll have finished his supper.
他该吃过晚饭了。(我肯定他吃过晚饭了。)
It's no use asking Tom;he won't know.
这事问汤姆没用,他不会知道。(我肯定他不知道。)
Will Bill be at the club now,do you think?
你觉得比尔现在会在俱乐部吗?(你认为他在吗?)
B should用于表示对现在或过去的行为的假设
should在此主要与现在式(仅限于表示未经预先考虑或计划的行为)的肯定式或否定式连用。有时也可与进行式或完成式连用:
The plane should be landing now.
现在飞机肯定在降落。(我认为飞机正在降落。)
The letter should have arrived by now.
信现在该到了。(我认为该到了。)
用should表示推断时没有用will表示推断时自信心强:
Tom should know the address.
汤姆应该知道地址。(我想汤姆知道。)
Tom will know the address.
汤姆会知道地址的。(我肯定他会知道的。)
He should have finished by now.
他现在该完成了。(我认为他该完成了。)
但是:
He'll have finished by now.
他现在想必完成了。(我肯定他完成了。)
should不用于表示使说话的人不高兴的推断:
Let's not go shopping now.The shops will be very crowded.
我们现在别去商店买东西了。商店这会儿人肯定特别多。(在此不必用should。)
但要表示与此相反的而使说话人高兴的推断时,两种形式都可以用:
Let's go shopping now.The shops will be/should be fairly empty.
我们去商店买东西吧。商店里人不会太多。
The shops won't be/shouldn't be too crowded.
商店里不会太拥挤。
C will和should也都可以表示对将来的假设:
He should/will have plenty of time to get to the station.
他该有足够的时间去火车站。