情態(tài)動詞 (Model Verbs) 又稱為情態(tài)助動詞 (Model Auxil-iaries).英語中助動詞主要有兩類：一是基本助動詞,如have, do, be；二是情態(tài)助動詞,如may, must, need等.情態(tài)動詞與其他動詞連用表示說話人的語氣.情態(tài)動詞可表達建議、要求、可能和意愿等.情態(tài)動詞沒有人稱和數(shù)的變化.在大學英語四、六級考試中,情態(tài)動詞部分重點測試以下內(nèi)容：
（1）情態(tài)動詞+行為動詞完成式
（2）情態(tài)動詞+行為動詞進行式
（3）情態(tài)動詞+行為動詞完成進行式
（4）某些情態(tài)動詞的特殊用法
一、情態(tài)動詞+行為動詞完成式
情態(tài)動詞+行為動詞完成式即“情態(tài)動詞+ have + v-ed分詞”,表示對過去行為或動作進行推測、評論或判斷.
1. must have v-ed
must have v-ed 表示推測過去某事“一定”發(fā)生了.其否定形式為：can’t / couldn’t have v-ed, 表示過去不可能發(fā)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 表示推測過去某動作“很可能”發(fā)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 表示推測過去某事“也許”發(fā)生了.may 比 might 表示的可能性在說話人看來稍大些.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 用于對已發(fā)生的情況表示“責備”、“不滿”,分別表示“本應(yīng)該…”和“本不應(yīng)該…”.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 表示過去做了某事,但沒有做的必要, 意為“本沒必要…”.例如：
You needn’t have waken me up; I don’t have to go to work today.
注：表示推測過去某動作發(fā)生的可能性時,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、情態(tài)動詞+行為動詞進行式
情態(tài)動詞+行為動詞進行式（即情態(tài)動詞+ be + v-ing形式）,表示推測或評論某動作現(xiàn)在是否正在進行.例如：
1）He must be playing basketball in the room.
2）She may be staying at home.
三、情態(tài)動詞+行為動詞完成進行式
情態(tài)動詞+行為動詞完成進行式（即情態(tài)動詞+ have been + v-ing 形式）,表示推測或評論過去某動作是否正在進行或一直在進行.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、某些情態(tài)動詞的特殊用法
1. need
考試中主要測試 need 作情態(tài)動詞與作實義動詞的區(qū)別,對此,可參見表1.need 作情態(tài)動詞時,后面跟不帶to 的動詞不定式,即 “need to”.通常用在疑問句和否定句中,表示詢問是否有“必要”,其否定形式為needn’t,表示“不必”；疑問形式為 Need … do? 極少用于肯定句.例如：
1）I don’t think we need turn the light on at that time.
2）Need you ride a bike to school?
情態(tài)動詞need與實義動詞need 在時態(tài)、肯定、否定結(jié)構(gòu)上的對比見表1.
2. dare
考試中主要測試dare作情態(tài)動詞和作實義動詞的區(qū)別.
情態(tài)動詞dare 與實義動詞dare 在時態(tài)、肯定、否定、疑問結(jié)構(gòu)上的對比見表2.
3. can 和 may
考試中主要測試can,may或could,might表示可能性的區(qū)別及對may構(gòu)成的疑問句的回答.

時態(tài)情態(tài)動詞need 實義動詞 need
現(xiàn) You need (not) doYou (don’t) need to do
在
時 He need (not) do He needs (doesn’t need) to do

過You needed (didn’t need) to do
去
時He needed (didn’t need) to do

將You need (not) do You will (not) need to do
來
時He need (not) doHe will (not) need to do




句型時態(tài) 動詞
情態(tài)動詞dare 實義動詞 dare
肯定句現(xiàn)在時dare to 少用 dare/dares to do
過去時dare to 少用 dared to do
否定句現(xiàn)在時daren’t/dare not dodo/does not dare (to) do
過去時dared not do did not dare (to) do
疑問句現(xiàn)在時Dare he do? Do you/Does he dear (to) do?
過去時Dared he do? Did he dare (to) do
（1）can, could, may, might都可以表示可能性.can, could表示潛在的可能性或理論上或邏輯判斷上存在的可能性；而may, might則表示事實上的可能性.此外,can還具有“有能力”的意思,而may與might則不具此意.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 …?這一類疑問句的肯定回答為Yes, please.或Certainly；否定回答為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 與 be able to 都可以表示能力,但兩者在用法上有點差異：can (could)表示主觀能力,不表示意愿,它的將來時用will be able to; be able to表示主觀意愿,強調(diào)要克服困難去做某事.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“必須”,但有幾點區(qū)別：
（1）must 強調(diào)“內(nèi)在的職責”、“義務(wù)”,而have to 強調(diào)“外界壓力”、“不得已而為之”.
（2）have to可用于多種時態(tài),而must一般用于現(xiàn)在時,其過去時與將來時分別由had to與shall / will have to代替.
（3）在回答must引起的問題時,如果是否定的答復(fù),不能用mustn’t,而要用needn’t或don’t have to,因為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意為“過去常常”,“過去一直”；be used to +v-ing / n（名詞）意為“習慣于”；be used to +v意為“被用來（做某事）”.
（2）used to只表示過去,而be used to +v-ing / n可表示現(xiàn)在、過去或?qū)?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．用作情態(tài)動詞的其他短語
would rather, would sooner, would (just) as soon, had rather, had better, had sooner, can not but, may (just) as well等可用作情態(tài)動詞.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：這些短語后一般直接跟動詞原形.would (had) rather, would (had) sooner, would (just) as soon后可跟that 引導的從句,that 常省去,從句要用虛擬語氣.對現(xiàn)在和將來的假設(shè)用過去時,對過去的假設(shè)用過去完成時.例如：
1）I would rather you came on Sunday.
2）I would sooner you hadn’t asked me to speak yesterday.