need做情态动词的时候和should做情态动词时的区别

Question

need做情态动词的时候和should 做情态动词时的区别

Answer 1

Should表示劝告、建议、命令,其同义词是ought to；在疑问句中,通常用should代替ought to.如：
You should go to class right away.
Should I open the window?
Should的含义较多,用法较活,现介绍三种其特殊用法.请看下面的句子：
① I should think it would be better to try it again.我倒是认为最好再试一试.
② You are mistaken,I should say.依我看,你是搞错了.
③ I should advise you not to do that.我倒是劝你别这样做.
④ This is something I should have liked to ask you.这是我本来想问你的.
从以上例句可以看出：情态动词should用于第一人称时可以表示说话人的一种谦逊、客气、委婉的语气.
Should还可以用在if引导的条件从句,表示一件事听起来可能性很小,但也不是完全没有可能.相当于“万一”的意思.从句谓语由should加动词原形构成,主句谓语却不一定用虚拟语气.如：
⑤ Ask her to ring me up if you should see her.你万一见到她,请让她给我打个电话.
⑥ If you should change your mind,please let us know.万一你改变主意,请通知我们.
⑦ Should I (If I should) be free tomorrow I will come.万一我明天有时间,我就来.
此外,Why(or How) + should结构表示说话人对某事不能理解,感到意外、惊异等意思.意为“竟会”.如：
⑧ Why should you be so late today?你几天怎么来得这么晚?
⑨ — Where is Betty living?— 贝蒂住在哪里?
— How should I know?— 我怎么会知道呢?
⑩ I don't know why you should think that I did it.我真不知道你凭什么认为这件事是我干的.
5.“should + have + 过去分词”结构一般表示义务,表示应该做到而实际上没有做到,并包含一种埋怨、责备的口气.如：She should have finished it.
I should have helped her,but I never could.
You should have started earlier.
一、need表示“需要”或“必须”,通常用于否定句,疑问句,条件句.例如：
1．You needn't do it again．你不需要重复做了.
2．He needn't worry about it．这件事他无需担心.
3．Need he do this homework first?他需要先做这些作业吗?
4．Need they fill in the form?他们需要填表吗?
二、在否定句中,可以用need的否定形式＋不定式完成体.例如：
1．We needn't have worried．其实我们不必要慌.
2．You needn't have mentioned it．你本来不必提起这件事.
3．You needn't have said that when he asked．当他问的时候,你其实不必要说.
三、needn't后的不定式间或也能用进行式或被动语态.例如：
1．He needn't be standing in the rain．他不必要站在雨中.
2．We needn't be waiting in this place．我们不必要在这儿等.
3．The hedges needn't be trimmed thisweek．本周树蓠不必要整修.
四、其一般疑问句的答语,肯定用must,否定用needn't.
Need I go with her?我需要和他一起去吗?
Yes,you must.是的,你需要去.
No,you needn't.不,你不必去.