情态助动词的词法特征是，没有非限定形式，没有词形屈折变化 句法特征是，协助主动词构成限定动词词组，无一例外地带有不带 to 的不定式；在限定动词词组中，只能用一个情态助动词，而且总是位居第一,情态助动词多半一词多义；在肯定句、否定句或疑问句中，往往意义不尽相同；而且其“时”的特征并不是时间区别的主要标志，也就是说，现在时清态助动词不一定表示现在时间，过去时清态助动词也不一定表示过去时间，各有其用法上的特殊性。
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情态助动词的词法特征是，没有非限定形式，没有词形屈折变化 句法特征是，协助主动词构成限定动词词组，无一例外地带有不带 to 的不定式；在限定动词词组中，只能用一个情态助动词，而且总是位居第一,情态助动词多半一词多义；在肯定句、否定句或疑问句中，往往意义不尽相同；而且其“时”的特征并不是时间区别的主要标志，也就是说，现在时清态助动词不一定表示现在时间，过去时清态助动词也不一定表示过去时间，各有其用法上的特殊性。
1) 表示“能力”
表示“能力' (Ability)' 可用 can, could, be able to。
can 既能表示现在的“能力”，也能表示将来的“能力＂。
Look! I can do it.
I can't do it now, but I can do it later.
can 表示现在的“能力”，既可表示做某桩具体事情的“能力”。
I can climb this cliff.
Is there anything I can do for
也可泛指一般的“能力' 。
He can drive a car.
He is only four, but he can read.
She can't play the piano.
Fire cannot destroy gold.
can 表示“能力”与 be able to 同义，在许多情况下可以交替使用。
As I have got enough money, I can /
尤其是将来的“能力”，通常用 will / shall be able to表示。
I'II be able to speak German in another two months.
If he doesn't apply fairly soon, he won't be able to get the
当然，要表示将来做某桩具体事情的能力也可用 can。
Can you go and see him tomorrow morning?
If I can, I will do it.
表示过去的“能力”，可用 could /was / were able to, 但在肯定旬中， could 表示的“能力”仅是泛指过去的一般能力。
She could play the piano when she was only six.
At that time I could still read without spectacles.
如果要表示过去做某桩具体事清的“能力”，通常不用 could, 而用 was / were able to (以及managed
He was able to swim halfway before he collapsed.
He was able to translate the article without a dictionary.
I talked a long time, and in the end I managed to make her believe me.
上述 could /
I couldn't / wasn't able to play the piano when I was at school.
He got so drunk that he couldn't / wasn't able to find the front door.
表示“能力”，除用 can, could, be able to 外，还有其他表示法。
He has the ability to deal with complicated problems.
You are capable of better work than this.
He is capable of running a mile in four minutes.
You have the capability to do/of doing this
He has great capabilities as a writer.
2) 表示“可能＇
表示“可能' (Possiity), 可用 may, might, can, could，
It may /
He may /
He may not /
也可表示将来的'可能' 。
He may /
He may not / might not leave next week.
在疑问句中表示“可能”，须用 can 而不用 may。
Where can he be? 哪里找得到他哟？
He may be in the office.他可能在办公室。
Can they have missed the bus?
Yes, they may have.
may not 表示“不可能”，重音通常落在助动词上。
He'may not go tomorrow. = It is possible