情态动词。
1.表示能力时，只能用can (现在) could do (过去能做)
was/were able to do ：
1） 过去能做
2 ) 过去设法成功完成某一具体动作 =manage to do
=succeed in doing
eg. blind man can not judge colors.
could/was able to swim like a fish when he was young.
he was tired , he was able to swim across the river.
he was tired, he couldn’t /wasn’t able to swim across the river.(在否定结构中无区别)
2.表推测时：may/might( 语气最不肯定，多用于肯，否定句) 可能。
can （语气肯定，一般不用于肯定句）可能。
could (较can不确定，也可用于肯定句) 可能
must (语气最肯定，只用肯定句)一定。
should(表可能性，推论，推测，期待等)按理该；估计。
ought to (表可能性，揣测，猜测，期望等)照说应该，理应。
may/might h**e +ed (语气不肯定) 过去可能……
can’t/couldn’t h**e +ed (语气肯定)过去不可能……
must h**e +ed (语气肯定) 过去一定。
should h**e +ed 过去按理应该。
ought to h**e +ed 过去按理。
eg. look, someone is coming. who can it be?
b: it may be the headmaster.
a:it can not be has gone to shanghai.
b: it must be mr zhang. he looks like the headmaster.
don’t know his age. he could be thirty or thirty-five.
3. the report is written after careful investigation, so it
should be reliable.
4.--i didn’t see mary at the meeting yesterday.
she might h**e been ill, i guess.
street are all dry. it couldn’t h**e rained during the night.
3. 表责备时：
may/might h**e +ed（语气委婉）过去本可能…
can/could h**e +ed (语气委婉) 过去本能……
should h**e +ed (语气强烈) 过去本应该……，而未……
ought to h**e +ed (语气强烈) 过去本应该……，而未……
needn’t h**e +ed 过去本不必…，却做了…
eg. might h**e given you more help, even though he was very busy.
2.--did you listen to the speech?
no, we could h**e attended we had a lot of traffic on our way.
plant is dead. maybe i should h**e given more water.
brother’s failed in english again. you ought to h**e given him more help.
5. -mr smith didn’t come last night, did he?
no. we needn’t h**e waited for him. a whole night was wasted.
4.表未曾实现：
was/were to h**e +ed 过去本打算/本想……
should like /love to h**e +ed 过去本想…… 而没……
would like/love to h**e +ed
should h**e liked /loved to do
would h**e liked/loved to do
had better h**e +ed 当初最好……
would rather h**e +ed 当初真该……
eg. 1. he was to h**e come here yesterday, but he had an important meeting to attend.
2. we would like to h**e gone to see the film, but we had no time.
3. i would love to h**e gone to the cinema to see the film that day, but the tickets were all sold out.
4. i should h**e liked to stay another week, but i had to go back to work.
had better h**e bought (当时真该买) the book.
would rather h**e stayed (当初真该呆) at home than go to see the dull film.
5. 委婉的请求肯定的答语。
couldcan
mightmay
wouldwill
eg. 1. -could you lend me your book?
certainly, i can.
2. -might i ask you a question?
surely, you may.
3. -would you do me a f**or?
of course, i will.
4. -would you like to do it for me?
yes, i’d like to. (特殊)
7. shall : 可用于。
一、三人称表示征询对方意见或请求指示。
eg. shall he wait for you at the gate?
shall we begin our lesson?
shall 用于第。
二、第三人称， 表示说话人对对方的命令、警告、允诺或威胁。
he shall be punished. (威胁)
you shall h**e the book when i finish reading.（允诺）
you shall fail if you don’t work harder. (警告)
:作情态动词时，主要用于疑问句、否定句和条件从句中，一般不用于肯定句。eg. how dare you say i’m unfair!
he daren’t speak english before such a crowd, dare he?
dare作实义动词时，在肯定句中，其后通常接带to的不定式；在否定和疑问句中，其后可接带to或不带to的不定式。
dare to swim across this river.
he does not dare (to) answer.
9. need 表示“需要”或“必须”，作情态动词时，仅用于否定句或疑问句中。在肯定句中一般用 must,h**e to, ought to 或should 代替。
needn’t come so early.
---need i finish the work today?
---yes, you must.
to 表示过去的习惯动作或状态（现在已不复存在），在间接引语中，其形式可不变。
told us he used to play football when he was young.
在疑问句、否定句、否定疑问句或强调句中，可有两种形式：疑问句。
did you use to go to the same school as your brother?
used you to go to the same school as your brother?
否定句：i usedn’t to go there.
i didn’t use to go there.
usedn’t 亦可拼作usen’t
否定疑问句：usen’t you to be interested in the theatre?
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