`can、could、may、might、should、ought to、must、have to、need、don't have to、had better、would rather、used to + v、would + v`
# can、could
* 表示能力，用`be able to`代替can/could现在/过去的能力
* 客观可能性（can的可能性更大）
* 表示请求和允许（could更委婉一些）
```
He can/could/is able to swim. 他能游泳。
He can/could come tomorrow. 他明天能（可能性）来。
Can/Could I stay here? 我能呆在这吗？
```
# may、might
* 表示可能性（may的可能性更大）
* 表示使请求和允许（might更委婉一些）
```
-Yes,please.
-No,you can't/mustn't（禁止，不准）
He may/might come here by bus.
May/Might I join you?
-Yes,please./No,you can't./No,you mustn't.
```
# must、have to
* 表示必须、必要
* must表示主观多一些
* have to表示客观多一些，有“不得不”的意思
* have to有时态和数量的变化
* must和have to二者的否定意义大不相同
* You mustn't go.你不准去。
* You don't have to go.你不必去。
```
You must get up early.你必须得早点起床。（主观色彩的必须）
It's going to rain,I have to go home now.外面要下雨了，我必须得回家了。（外界的客观原因，我不得不回家）
```
# should、ought to
* 表示劝告、建议、命令
* should强调主观看法
* ought to强调客观要求
* 在疑问句中，通常用should代替ought to
```
You should/ought to do the job right now.
Should they stay here now?
```
# need、don't have to
## need 需要
* 情态动词
```
He need come here early.
He needn't come here early.
Need he come here early?
-Yes,he need./No, he needn't.
```
* 实意动词，有第三人称单数和时态的变化，后面可加to的不定式
```
He needs to come here early.
He doesn't need to come here early.
Does he need to come here early?
-Yes,he does./No,he dosen't.
```
## don't have to 不需要，不必
回答must和have to的提问句时，否定式使用needn't/don't have to等回答方式
```
Must I come here early tomorrow?
-No,you needn't/don't have to.
```
# had better、would rather
* had better 表示“最好做某事”，had虽然是过去式，但不表征过去式，better后面接动词原形
```
He had better eat more.
You'd better finish it right now.
```
* would rather表示“宁愿、宁可、最好、还是······为好”，语感上比`had better`要轻
```
You would rather deal with it now.
```
* 否定形式分别是：
* had better not + 动词原形
* would rather not + 动词原形
```
He had better not eat more.
You would rather not deal with it now.
```
# used to + v、would + v
均表示过去习惯性的动作，可翻译为：”过去常常···“
* used to 可指过去的状态或情况，would则不能
```
The novel used to be popular.那个小说过去很流行。
```
* would表示反复发生的动作。如果某一动作没有反复性，就不能用would，只能用`used to`
```
He would practise English every week.他过去每周都练习英语。
I used to live in Beijing.我过去住在北京。
```
* used to表示过去经常性或习惯性的动作或状态现在已经结束，would则表示有可能再发生
```
People used to believe that the earth was flat.人们过去认为地球是平的。
He would go to the park as soon as he was free.他只要一有时间就常常去公园。
```
