情态动词属于非实意动词，它虽具有一定的词义，但这种词义并不完整，语法上也不能单独构成谓语，它必须和另一个实意动词一起构成谓语才能表达完整的意思。情态动词是一类非常活跃的非实意动词，归纳起来，情态动词在句子中主要有三种用法：
1．情态动词的非推测性用法
其用法是用来表示能力、许可、必要、义务、命令、劝告、询问、需要、应该、敢于等概念或态度，这类情态动词有：can（could），may（might），shall（should），will（would），must，
oughtto，need，dare等，例如：
Children，you can eat something you like．
Everybody must pay the fare，but you need not do so right away．
How dare you come out without your parents'permission？
I would rather stay at home than go with you．
Ought not parents to give their child more care？
You are not so young；you should not do such silly things any more．
疑问句中的情态动词和答语中的情态动词有固定的搭配形式。例如：
-Could I borrow your bike for a while？
-Yes，you can．（-Yes，please．）
-No，you can not．
-May I discuss something on practicing spoken English with you now？
-Yes，you may．（-Yes，of course．）
-No，you mustn't．
-Need everyone fill in this application form immediately？
-Yes，everyone must．（Yes，everyone do．）
-No，they needn't．
-Must we hand out emergency rations to these famine refugees this instant？
-Yes，you must．
-No，you they needn't．
-No，you don't have to．
2．情态动词的推测性用法
人们常常会对客观事物做出自己的判断，当情态动词用来表示说话人对事物的推测、判断时，就具有情态动词的推测性用法。可以用作推测的情态动词有can，could，may，might，must，ought to， should，will和would。它们做出可能性的推测、判断、预料的程度是不同的，might最弱、最没有把握，而must最强、把握最大。例如：
It's nine o'clock．My friend couldn't come to see me．
This boy must fail in exam from his sad face．
It's five past eight．There ought to be many students waiting for the door opening．
She should be a clerk in foreign company according to her formal dress．
He may not be fine in these days．
They were ready for a strong earthquake which might occur．
有时，情态动词用不同的动词时态来推测表达对不同时间的事物的推测，例如：
My friend may be still sleeping at home now．（对现在说话时正在发生的事物的推测）
Can we be working at the same company next year？（对将来的推测）
His clothes is wet．He must have forgotten to bring the umbrella．（对已经完成的情况的推测）
3．情态动词用于虚拟语气
A：提出一个假设的条件，纯粹的展望或提出命令、要求。有部分情态动词的过去时态常用于这种用法。例如：
Wish I might be better soon．
Should my friend come，ask him to wait in the reading-room．
May you be healthy forever．
If you dared to say such words to me，I would give up making friends with you．
B：比较委婉地陈述看法，提出要求和建议，谓语动词通常是情态动词的过去时态。例如：
It would not be useful to adopt such suggestion．
Could you leave the message with me？
C：情态动词加动词的完成时态用来表示对过去情形的虚拟，这种设想或看法同过去的事实是相反 的，表示"本应该做"，而实际上没做。例如：
We ought to have been here early．
This is something you shouldn't have done．
Need you have nosed about in her affair？It was none of your business．
- 考试辅导
