(1) 当说某事可能时，有两种含义。
- It's possible to hold one's breath for three minutes, with practice.
通过训练，一个人可能屏住呼吸达三分钟。 - A: Do you think you'll come back next year?
你认为你明年能回来吗？
B: It's quite possible.
很有可能。
＊第一例中只谈此事发生的理论上的可能，不涉及此事实际发生的可能性；第二例中说及某事实际发生的可能性。两种情况语法上相似，意思却不同。
＊can常用来表达第一种可能性——理论上的可能性。
- Anybody who wants to can become a visitor here.
只要你愿意，就可来参观。
(2) 当要表达某事发生或变成真实的实际可能性时，则须用could, may，might。
- Will you answer the phone? It could/may/might be your mother.
你接一下电话好吗？可能是你母亲打来的。 - According to the radio, it could/may/might rain tonight.
根据广播内容，今夜可能下雨。
(3) 含有can的句子常用来表示人的行为或事物变化的特征。
- Mary sometimes can be very naughty.
玛丽有时很调皮。 - Scotland can be very warm in September.
苏格兰9月份可能会很暖和。 - Gold can be dissolved in hydrochloric acid.
黄金会在盐酸中溶化。
(4) 如要表示过去某人性情行为等或事物变化的这种可能性，则要用could。
- My grandfather could be very unpleasant at times.
我祖父有时会让人非常不愉快。
(5) can 常用来就一个问题可能的解决方法或可能采取的行动提出建议。
- We've got three choices: we can go to the police, we can talk to Peter ourselves, or we can forget all about it.
我们有三个选择：可以去找警察，可以亲自找彼得谈，再不，就把它全忘了。 - A: What shall we do?
我们该怎么办？
B: We can try asking Lucy for help.
我们可以试试请露西帮忙。
(6) 为使建议语气和缓——不那么强或不那么肯定，可用could。
- We could try asking Lucy for help, if you think it's a good idea.
如果你认为这是个好主意，我们可以试试请露西帮忙。 - Could you come to my place tomorrow morning to discuss how to make out the examination papers?
你明天上午能到我这儿来讨论一下如何出试卷吗？
(7) 有时，这种“建议”实际上是要求或命令。
- You could give me a hand with the cooking.
你可以帮我做做饭。
(8) could有时表示不太有把握的可能性。
- We may go climbing this summer. (把握性较大)
今年夏天我们可能去爬山。
We could go climbing this summer, but I doubt if we'll have time. (不太肯定)
今年夏天我们可能去爬山，不过我怀疑我们是否有时间。
- It could rain later on this evening.
今晚晚些时候可能要下雨。
(9) 如要表示现在(说话之时)某事可能是真实的，可用may, might和could。
- You may be right.
你可能是对的。 - You could be right, but I don't think you are.
你也可能对，但我并不这么认为。 - This could be your big chance.
这可能是你的一次大机会。
(10) can有时可表示现在的可能性，但只在疑问句和否定句中这样用。
- Who can that be at the door?
门口那人会是谁？ - What can she possibly want?
她可能会要什么呢？ - Can it be Susan?
那会不会是苏珊？ - It can't be true.
这不可能是真的。
＊注意： 在这个意义上，can't是must的反义词：
- It can't be true. She must be mistaken.
这不可能是真的，准是她弄错了。
(11) can, could均可与不定式的完成时(have+p.p.)连用，表示对过去情况的猜测，这时，can只用于疑问句和否定句。
- Where can she have gone ? She can't have gone to school — it's Saturday.
她会去哪儿呢？她不可能去上学——今天是星期六。 - She could have gone off with some friends.
她可能跟一些朋友走了。
(12) could与不定式完成时连用也可表示过去未实现的可能性——某事有可能，但并未发生。
- You were stupid to go skiing there — you could have broken your leg.
去那里滑雪你真蠢——你会摔断腿的。 - It wasn't a good idea to throw the TV out of the window — it could have hit somebody.
从窗户往外扔电视机可不是个好主意——那可能会砸着人。
(13) 这种结构也可用来批评某人没做某件事，较“should+have+done”结构语气稍弱。
- You could have told me you had invited people to dinner. (Why didn't you tell me...?)
你邀请了人来吃饭，你本可以告诉我一下嘛。
(14) 如要表示过去某事不可能，可用“It couldn't have happened”的形式。
- My parents wanted me to be a doctor, but I couldn't have put up with all those years of study.
父母想要我成为一名医生，但我不可能去忍受那么多年的学习。
(15) may和might也可与不定式完成时连用，这点与could相似。
