一、情态动词的推测用法
许多情态动词用来表示推测,从最不肯定到最肯定,所表达的可能性,从小到大顺序大抵如下:
might
may 很不肯定
could
can
can / should
ought to
would
will
must 几乎可以肯定
You are right.肯定
(此表摘自Longman English Grammar， P.392 外语教学与研究出版社)
掌握这个表对于我们正确运用情态动词来表达“可能性”这个概念是有帮助的。例如,当你手拿一大串钥匙去开门,但你不知道究竟哪一把钥匙
能够把门打开,于是你试第一把,你说:“It might/may be the key to the door.”; 几乎所有的钥匙都试过了,恰好都不能把门打开,最后只剩下两把,你再试,你就可以说:“It must be the key.”
二、 can
can 可以用来表示:
A. 能力（ability）
He can run faster than I. 他能跑得比我快。(=He is able to run faster than I.)
Can you lift this box 你能抬起这个箱子吗?
B.允许(permission)
Can we stop working now 我们现在可以停止工作了吗?(=Are we allowed to stop working now)
You can sing a song in here. 你可以在这里唱歌。(=You are allowed to sing a song in here .)
You can't travel first-class with a second-class ticket. 你不可以拿二等车票,坐头等位子旅行。
C.学到的技能
Can you play bridge （=Do you know how to play bridge）你会玩桥牌吗?
D. 可能性(possibility)
Can she be writing me a letter 她可能在给我写信吗?
（=Is it possible that she is writing me a letter）
She can't be in the office now. I met her in the shop a moment ago. 她现在不可能在办公室里,刚才我还在商店里见到她。
请看1997年6月的四级第32题:
You cannot be careful when you drive a car.
A） very B） too C） so D） enough
你开车时越小心越好。(B对)
You can't be too careful when you drive a car. 里的can ,用的正是这一含义,该句相当于It's impossible that you are too careful
when you drive a car. 即:“你开车时太小心这种情况是不可能的。”
句型can not/never/hardly...too/enough... 表达强烈的肯定,常常译成“越……越好”,“再……也不过分”。例如:
You can't be careful in making the decision as it was such a critical case.
A） very B） too C）quite D） so
（B6， 1990.1-51题,B对）由于事关紧要,因此作决定时务必仔细斟酌。
We cannot stress the importance of the theory too much. 这一理论的重要性,怎样强调都不过分。
The important role people played in history can never berated too much.人民在历史上所起的作用,无论怎样评价都不会过高。
A book can hardly last too long. 好书定能永远流传下去。
The effect of science and technology on social proGREss can never be estimated enough. 科学技术对社会进步所产生的影响,无论怎样评价都不过分。
最后,can表示“可能性”时,隐含sometimes 的意思:
Swimming can be dangerous. = Swimming is sometimes dangerous. 游泳有时是危险的。
An adult can act as foolish as a little child. 大人有时也会像小孩那样愚蠢行事。
