情態(tài)動(dòng)詞的完成式：
can / may / should / ought to / must / need等動(dòng)詞稱為情態(tài)動(dòng)詞。情態(tài)動(dòng)詞有三大特征：一是無(wú)人稱和數(shù)的變化，二是后面跟的動(dòng)詞不帶"to"，三是后面 + not構(gòu)成否定式，比較：
He needn't go there so early. (他不需要去那么早。- 情態(tài)動(dòng)詞)
You do not need to be an expert to make this simple design. (這么簡(jiǎn)單的設(shè)計(jì)不一定非要專家來(lái)做。- 動(dòng)作動(dòng)詞)
在講解"情態(tài)動(dòng)詞完成式"前，先將一般式中應(yīng)注意的幾個(gè)問(wèn)題提一下，以引起必要的重視。
1.情態(tài)動(dòng)詞一般式中應(yīng)注意的幾個(gè)問(wèn)題：
注意以下用法的習(xí)慣與區(qū)別：
He must be a doctor. (他準(zhǔn)是個(gè)醫(yī)生。)
否定：He can't be a doctor. (他不會(huì)是個(gè)醫(yī)生。- 注意：一般不用must' be )
又如：They can't be working now.(現(xiàn)在他們不可能在工作。)
May I go now?
回答：Yes, please. / Please don't. (或：No, you mustn't)
have to 與 must
1) 當(dāng)強(qiáng)調(diào)"外界原因"時(shí)，2)當(dāng)與"將來(lái)時(shí)"結(jié)合時(shí)，用have to，
例如醫(yī)學(xué)全在.線.提供, bhskgw.cn：
Since it is raining, we have to sty indoors.
You will have to hurry if you want to catch the 9:30 train. (你要是趕9:30的火車，得趕緊。)
3) 表示自然規(guī)律或必然結(jié)果，用must，
例如：
All men must die. (人固有一死。)
can 與be able to
can表示能力，但不一定做了或做成了;be able to 表示有能力并且做成了，相當(dāng)于succeed in doing, 例如：
The patient is able to walk now. (說(shuō)明病人原來(lái)是喪失了走的能力的，現(xiàn)在自己能走了)
The patient can walk now. (可能的情況是，病人是能走的，但醫(yī)生不讓他走，如急性心梗后醫(yī)生不讓病人走動(dòng)。)
Shall I / we(第一人稱)…? 表示征求對(duì)方意見;(第二、三人稱) + shall…表示"允諾"或"警告，例如：
Shall I fetch a doctor for you? (要我為你請(qǐng)個(gè)醫(yī)生來(lái)嗎?)
You shall be sorry for what you have done. (你會(huì)為你所做的事后悔的。)
Will you….? 表示請(qǐng)求， 例如：
Don't' be late for the meeting next time, will you? (下次開會(huì)別遲到了，好嗎?)
比較：I would remind you that smoking is forbidden here. (表示"婉轉(zhuǎn)的建議或請(qǐng)求"：我想提醒你，這里不能抽煙。)
needn't 的肯定式為must，例如：
You needn't see him, but I must. (你不必去看他，但我得去。)
cannot but + 原形動(dòng)詞(意思相當(dāng)于have to)， 例如：
I cannot but tell you the truth. (我不得不給你說(shuō)實(shí)話。)
2.情態(tài)動(dòng)詞的完成式：
情態(tài)動(dòng)詞的一般式用于表達(dá)"現(xiàn)在"或"將來(lái)"的情況，而情態(tài)動(dòng)詞的完成式則用來(lái)表達(dá)"已經(jīng)發(fā)生的事情"，例如："你不應(yīng)該來(lái)得那么晚。"這是對(duì)已經(jīng)發(fā)生的事情(你來(lái)晚了)表示自己的看法：You shouldn't have come so late.
將常用、�？嫉膸讉�(gè)情態(tài)動(dòng)詞完成式列表說(shuō)明如下：
|
must （not) have done |
表示對(duì)已發(fā)生的事情的"判斷"，中文常表達(dá)為："準(zhǔn)是…" |
It must have rained last night; the ground is wet.（昨晚準(zhǔn)是下雨了，地上濕漉漉的。) |
|
should （not) have done / ought （not) to have done |
表示"事與愿違"，中文常表達(dá)為："（不)該…"，"本（不)該…" |
The baby was cut badly; she shouldn't （oughtn't to) have let him play with the scissors. （孩子傷得很厲害；她不該讓他玩剪刀。) |
|
may / can （not) have done |
表示"本（不)可能" |
She may have known the news; she is crying. （她可能已經(jīng)知道那個(gè)消息了；她在哭。) |
|
need （not) have done |
表示"本（不)需要" |
You needn't have bought that dictionary; I have got a spare one. （你實(shí)際上沒(méi)有必要買那本字典；我有一本多余的。) |
由于在沒(méi)有情景的情況下很難區(qū)分情態(tài)動(dòng)詞一般式和完成式，例如"你不該去"，究竟是You shouldn't go there. 還是 You shouldn't have gone there, 因此考試的時(shí)候往往會(huì)給出情景，這對(duì)判題非常重要，如上面表格例子中的"the ground is wet"，"the baby was cut badly"，"she is crying", 都是題目所給的情景。
