四、need和dare

      这两词既可做实义动词用，又可做情态动词用。作为情态动词，两者都用于疑问句，否定句和条件句，很少用于肯定的陈述。need 作实义动词时后面的不定式必须带to，而dare 作实义动词用时, 后面的to 时常可以被省略。
①need用作实义动词和其他普通的实意动词一样有时态、人称和数的变化，其否定式、疑问式要用do. need（需要, 要求）　need + n. / to do sth.
We need help him. (×)
②情态动词：need,只用原形need后加do，否定形式为need not。
——Need you go yet? ——Yes, I must. / No, I needn't.
He needs not leave at once. (×)
He needed not leave at once. (×)
——Need/Must I take the bus? ——Yes, you must.
③need用作实意动词的被动含义：need, want, require, worth（形容词）后面接doing 可表示被动：
sth. needs doing= sth.needs to be done.
④There is no need (for sb.)to do sth. 与It is no need doing sth.
⑤needn’t have done sth.与didn’t need to do sth.
They needn’t have pushed him so hard. (本来没必要但实际上逼迫他过分了)
They didn’t need to push him so hard. （实际上也没有逼他过分）

五、will和would

1. will表示主观意志、意愿，用于各人称。
If you’ll tell me about what you have in mind, I will help you.
2. 表示经常性、习惯性、倾向性，“经常、总是”。would用于表示过去的习惯。
Oil will float on water.
The door won’t open.
3. 表示命令或允诺
You will get necessary help.
4. 表示推测、估计，“可能、大概”，指现在或将来。可能性不如must大。would指现在或过去。
That will be my son knocking at the door. He said he would come here at this time.
That would be the pen you had been looking for.
5. 表示功能
The hall will seat over 300 people.
6. Will you…?　Would you like…? 表示肯定含义的请求、劝说时一般用some。
例如：Would you like some cake?
7. 否定结构中用will，一般不用would, won't you是一种委婉语气。
Won't you sit down?
would用于第二人称表示请求或征求意见，比will更客气。

六、shall, should

1. shall用于第一、第三人称疑问句，征求对方意见
2. shall用于第二、第三人称陈述句，表示命令、警告、威胁、允诺等。
He shall end up in prison if he steals like this.
3. 用于所有人称，表示规定、法令、预言等，意思为“必须”。
4. should（ought to）用于表示义务、责任、劝告、建议、命令等，“应该”
5. should have done sth.“本来应该…” shouldn’t have done sth.“本来不应该…”有时有责备的意思。
6. should表示有一定根据的推测或可能性，“应该会…”
He should have got there by now.
7. should在疑问句表示惊讶、不应该等。
8. 用于I should say/advise/think…,等，表示谦虚、婉转。（虚拟语气另讲）