英语语法之情态动词详解
北京家教中心： 语法现象中有那么一类动词需要大家特别的关注和理解。这就是情态动词，初识其语法特征：“情态动词+动词原形”作句子的谓语。混淆点就在于几组区分，详见以下语法内容的讲解。北师大家教网整理了相关资料，以供参考：
情态动词表示期待或估计某事的发生,没有人称、数的变化(即第三人称单数不加-s)。
1. 比较can 和be able to
can/could 表示有能力做某事;可能 (过去时用could)。
Eg. I can speak English fluently. I could swim when I was eight years old.
be able to 表示成功地做了某事时。例如：
Eg. He was able to flee Europe before the war broke out. = He managed to flee Europe before the war broke out. 他在战争爆发之前逃离欧洲。
注意：区别can 与could
提出请求时，could更委婉(注意在回答中不可用could)。
--- Could I have the television on? 我能看电视吗?
--- Yes, you can. / No, you can't. 可以/不可以。
表示猜测时，could多用于疑问句。That can’t be true. \ Could that be true?
2. 比较may和might
表示允许或请求; Eg .May I come in?
表示没有把握的推测; Eg . He may\might be at home. 他可能在家。
may 放在句首，表示祝愿。Eg . May God bless you! 愿上帝保佑你!
may/might as well，后面接不带to 的不定式，“不妨…”。
eg. If that is the case, we may as well try.如果情况确实如此，我们不妨试一试。
3. 比较have(has\had) to和must “'必须”
have to 表示客观的需要，must 表示说话人主观上的看法，主观上的义务必要。
eg. My brother was very ill, so I had to call the doctor in the middle of the night.
弟弟病得很厉害，我只得半夜里把医生请来。
He said that they must work hard. 他说他们必须努力工作。
注意：否定结构中：don’t have to表示“不必”，mustn’t表示“禁止”。
eg. You don't have to tell him about it. 你不一定要把此事告诉他。
You mustn't tell him about it. 你不得把这件事告诉他。
Must 提问，肯定回答用 must 或have to; 否定回答用needn’t或don’t have to.
eg. Must I sleep right now? Yes, you must\have to. No, you don’t have to.
must表示推测, 在肯定句中表示较有把握的推测，意为“一定”。(否定推测用can\'t，不能用musy’t。)
eg. You have worked hard all day. You must be tired.
注意：should 和ought to
表示“应该;想必一定，按理应该;”。两者常可互换。
eg. It ought to be ready now. 想必现在准备好了吧。
They should be there by now, I think. 现在他们该到那儿了。
The poems should be out in a month at most. 诗集估计最多还有一个月就要出版了。
Should还可表示“究竟是…,到底…(表示意外，纳闷，惊讶)”
Eg. Why should you think that way? 你到底为什么会这样想?
4. had better表示“最好”， (had better do \had better not do)
eg. It is pretty cold. You’d better put on my coat. 天相当冷。你最好穿上我的外套。
5. would rather表示“宁愿” (would rather do \would rather not do)
eg. I would stay here than go out.
6. will和would
1）表示请求劝说时用would like;would like to do = want to
eg. Would you like to go with me? 想和我一块去吗?
2）Would you like…?疑问句中一般用some, 而不是any。
eg. Would you like some cake? 吃点蛋糕吧。
7. 比较need和dare
这两词既可做实义动词用，又可做情态动词用。
作为情态动词，结构：need\dare do; needn not do\dare not do.
作实义动词时，结构：need to do\ don’t need to do; dare to do\don’t dare (to)do.
8. 情态动词表达推测
对现在或将来的推测用“情态动词+动词原形”。
肯定的推测：一般用must, should, may(might)或can(could), 其中must的语气最强，意为“肯定”;should语气其次，意为“很可能，应该”，指按常理推测;can(could)，意为“有时会”;may(might)语气最弱，意为“也许，可能”。
否定的猜测：语气不很肯定时，常用couldn’t, may not, might not, 意为“可能不，也许不”;否定语气最强时，用can’t, 意为“根本不可能，一定不能”。
对过去发生事情的推测，用“情态动词+ have +过去分词”。
1) may(might)have done sth 可能发生了某事
Eg.It was too late, I thought he may have gone to bed.
2) can(could) have done sth 本来能够做的但事实上没有做
Eg. ---A taxi wasn’t at all necessary, it is so near.
---Yes, we can\could have walk to the station.
3) can(could) not have done sth 不可能发生某事
Eg. I saw Mr. Wang just now, he can’t\could’t have gone to Beijing.
4) must have +done sth 肯定发生了某事
Eg. ---Linda has gone to work, but her bicycle is still here.
---She must have gone by bus.
5)should \ought to have done sth 本应该做某事，而事实上并没有做
Shouldn’t \ought not to have done sth本不该做某事却做了
Eg. You ought to (should) have been more careful in this experiment.
6) needn’t have done sth表示本没必要做某事，但做了。
Eg. I dressed very warmly for the trip, but I needn't have done so. The weather was hot.
7) would like to have done sth表示本打算做某事，但没做。
had better have done sth 表示与事实相反的结果，意为“本来最好”。
Eg. I would like to have read the article, but I was very busy then.
我想读这篇文章，但太忙了一直没读。
Eg. You had better have come earlier. 你本该来得早一点。
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