Unit 14 Zoology [知識點撥]
e.g. you must be very tired. 你一定很累。
（= i am sure that you are very tired.） ( 很有把握)
you may / might / could be very tired. 你可能很累吧！
(=perhaps / maybe you are very tired.)（不太有把握）
（2）、在否定句中，can't 則表示“不可能”，語氣最強，而相應地may not, might not 語氣弱些，表示“可能不”。
e.g. he can't be at home. 他不可能在家。（很有把握）
he may not / might not be at home. 他可能不在家。（不太有把握）
（3）、疑問句中，通常用can表推測。
e.g. — someone is knocking at the door.
— who can it be ? 可能是誰呢？
where can he be now? 他現在有可能在哪呢？
二、對過去的動作或狀況的推斷常用“情態動詞+have done”。語氣強弱與以上三點相同，可依次類推。但表示“過去不可能”時，除了可用can't have done之外還可用couldn't have done。
e.g. he must have gone to wuhan. （肯定已經去了）
he may / might / could have gone to wuhan. （可能已去了）
he can not / could not have gone to wuhan. （不可能已去了）
can he have gone to wuhan? （可能去了嗎？）
can(could)/may(might)/must/will等+have+done用來對過去發生的情況做出語氣強
弱不同的推測。具體使用時請注意以下幾點：
① can一般用于疑問句和否定句, 極少用于肯定句。 e.g. ---can she have gone to school? ---no, she can't have gone to school. i saw her just now.
② may表推測，一般不用于疑問句；疑問句中該用can, could或might。 e.g. how could he have forgotten such an important thing? （不用may） might you have met him somewhere? （不用may）
③ could, may, might 用于肯定句語氣較弱，must用于肯定句語氣很強; may,
might 用于否定句語氣較弱，can, could 用于否定句語氣很強。 e.g. he could/may/might have gone to school. 他可能上學去了。 he must have gone to school. 他肯定是上學去了。 he may/might not have gone to school. 他也許沒去上學。 he can't/couldn't have gone to school. 他肯定沒去上學。
④ must表推測，一般不用于否定句；否定句中該用can或could。
誤：we mustn't have met before.
正：we can't have met before. needn't + have + done 可用來表示“已做了多余的事”。 e.g. there is no school today. you needn't have come.
could + have + done 可用來表示沒做某事的“遺憾”。 e.g. she came here on foot, but she could have come by bus.
could / might + have + done 也可用來表示委婉的批評。 e.g. you could have come 5 minutes earlier. you might at least have written me a letter.
在反意問句中，表推測的情態動詞不出現在簡短問句部分,取而代之的應是情態動詞。句中有過去時間狀語的，問句部分用過去時，否則，用現在完成時。