情态动词(Model Verbs)又称为情态助动词(Model Auxil-iaries)。英语中助动词主要有两类：一是基本助动词，如have,do, be；二是情态助动词，如may, must, need等。情态动词与其他动词连用表示说话人的语气。情态动词可表达建议、要求、可能和意愿等。情态动词没有人称和数的变化。在大学英语四、六级考试中，情态动词部分重点测试以下内容：
(1)情态动词 行为动词完成式
(2)情态动词 行为动词进行式
(3)情态动词 行为动词完成进行式
(4)某些情态动词的特殊用法
一、情态动词 行为动词完成式
情态动词 行为动词完成式即“情态动词 have v-ed分词”，表示对过去行为或动作进行推测、评论或判断。
1. must have v-ed
must have v-ed表示推测过去某事“一定”发生了。其否定形式为：can’t / couldn’t have v-ed,表示过去不可能发生某事。例如：
1)Since the ditch is full of water, it must have rained last night.
2)You couldn’t have met my grandmother. She died before you were born.
2. could have v-ed
could have v-ed表示推测过去某动作“很可能”发生了。例如：
1)“The dictionary has disappeared. Who could have taken it?”
2)“Tom could have taken it. He was alone yesterday.”
3.may / might have v-ed
may / might have v-ed表示推测过去某事“也许”发生了.may比 might表示的可能性在说话人看来稍大些。例如：
I can’t find my keys. I may / might have left them at the school yesterday.
4. ought to / should have v-ed和 ought not to / shouldn’t have v-ed
ought to / should have v-ed和 ought not to / shouldn’t have v-ed用于对已发生的情况表示“责备”、“不满”，分别表示“本应该…”和“本不应该…”。例如：
1)With all the work finished, I should have gone to the party last night.
2)You ought not to make fun of him. He is not the one you laugh at but learn from.
5. needn’t have v-ed
needn’t have v-ed表示过去做了某事，但没有做的必要，意为“本没必要…”。例如：
You needn’t have waken me up; I don’t have to go to work today.
注：表示推测过去某动作发生的可能性时，就表示的可能性程度而言，must最大，could其次，may更次之，might最小。例如：
“I wonder how Tom knew about your past.”
“He must / could / may / might have heard of it from Mary.”
二、情态动词 行为动词进行式
情态动词 行为动词进行式(即情态动词 be v-ing形式)，表示推测或评论某动作现在是否正在进行。例如：
1)He must be playing basketball in the room.
2)She may be staying at home.
三、情态动词 行为动词完成进行式
情态动词 行为动词完成进行式(即情态动词 have been v-ing形式)，表示推测或评论过去某动作是否正在进行或一直在进行。例如：
1)They should have been meeting to discuss the problem.
2)He may / might have been buying stamps in the post office when you saw him.
四、某些情态动词的特殊用法
1. need
考试中主要测试need作情态动词与作实义动词的区别，对此，可参见表1.need作情态动词时，后面跟不带to的动词不定式，即“need to”。通常用在疑问句和否定句中，表示询问是否有“必要”，其否定形式为needn’t，表示“不必”；疑问形式为Need … do?极少用于肯定句。例如：
1)I don’t think we need turn the light on at that time.
2)Need you ride a bike to school?
情态动词need与实义动词need在时态、肯定、否定结构上的对比见表1。
2. dare
考试中主要测试dare作情态动词和作实义动词的区别。
情态动词dare与实义动词dare在时态、肯定、否定、疑问结构上的对比见表2。
3. can和 may
考试中主要测试can，may或could，might表示可能性的区别及对may构成的疑问句的回答。
时态情态动词need实义动词need
现You need (not) do You (don’t) need to do
在
时He need (not) do He needs (doesn’t need) to do
过You needed (didn’t need) to do
去
时He needed (didn’t need) to do
将You need (not) do You will (not) need to do
来
时He need (not) do He will (not) need to do
句型时态动词
情态动词dare实义动词dare
肯定句现在时dare to少用dare/dares to do
过去时dare to少用dared to do
否定句现在时daren’t/dare not do do/does not dare (to) do
过去时dared not do did not dare (to) do
疑问句现在时Dare he do? Do you/Does he dear (to) do?
过去时Dared he do? Did he dare (to) do
(1)can, could, may, might都可以表示可能性.can, could表示潜在的可能性或理论上或逻辑判断上存在的可能性；而may, might则表示事实上的可能性。此外，can还具有“有能力”的意思，而may与might则不具此意。例如：
1)According to the weather forecast, it may rain tomorrow.
2) Any man with a little sense can see that he is wrong.
(2)May I / we …？这一类疑问句的肯定回答为Yes, please.或Certainly；否定回答为Please don’t.或No, you mustn’t.例如：
“May we leave now?” “No, you mustn’t. You haven’t finished your home work yet.”
4. can和 be able to
can与 be able to都可以表示能力，但两者在用法上有点差异：can (could)表示主观能力，不表示意愿，它的将来时用will be able to; be able to表示主观意愿，强调要克服困难去做某事。例如：
1)My grandma is over seventy, but she can still read without glasses.
2)He is able to give up his bad habits.
5. must和 have to
must和 have to都可以表示“必须”，但有几点区别：
(1)must强调“内在的职责”、“义务”，而have to强调“外界压力”、“不得已而为之”。
(2)have to可用于多种时态，而must一般用于现在时，其过去时与将来时分别由had to与shall / will have to代替。
(3)在回答must引起的问题时，如果是否定的答复，不能用mustn’t，而要用needn’t或don’t have to，因为mustn’t是“一定不要”、“一定不能”的意思。例如：
1)You must come to the classroom before eight.
2)It is raining hard outside, but I have to leave now.
3)“Must we do it now?” “No, you needn’t.”
6. used to v, be used to v-ing和be used to v
(1)used to v意为“过去常常”，“过去一直”；be used to v-ing / n(名词)意为“习惯于”；be used to v意为“被用来(做某事)”。
(2)used to只表示过去，而be used to v-ing / n可表示现在、过去或将来。例如：
1)He used to smoke. Now he doesn’t.
2)He’s quite used to hard work / working hard.
3)The knife is used to cut bread.
7.用作情态动词的其他短语
would rather, would sooner, would (just) as soon, had rather, had better, had sooner, can not but, may (just)as well等可用作情态动词。例如：
1)The soldier would sooner die than surrender.
2)The brave soldier would as soon die as yield to such an enemy.
3)I’d rather walk than take a bus.
4)If you don’t like to swim, you may just as well stay at home.
注：这些短语后一般直接跟动词原形.would (had) rather, would (had) sooner, would (just) as soon后可跟that引导的从句，that常省去，从句要用虚拟语气。对现在和将来的假设用过去时，对过去的假设用过去完成时。例如：
1)I would rather you came on Sunday.
2)I would sooner you hadn’t asked me to speak yesterday.
