帮忙解释一下should 与shouldn't 的含义及用法

Question

它和 must 还有 need 有什么区别?

比如 John is very sick. He should see a doctor.
也可以说成 He must see a doctor . He need see a doctor. 吧
有什么区别?

Answer 1

汉语翻译吧，should是“应该”的意思，比如你举的例子，John病得很严重，他“应该”去看医生了。

也可以说He has to see a doctor，这里的“has to”有不得不的意思。must是“必须”的意思，有种主动的含义，这里用“should”（应该，应当）比较好。

下一句有点小小的语法错误，应该是He needs to see a doctor，意思是，他“需要”去看医生，语气没有“should”重。

Answer 2

he should see a doctor他应该看医生，强烈建议
he must see a doctor他非看医生不可，刻不容缓，没有商量余地
he needs to see a doctor他需要看医生，和第一句相近，但出发点略有不同。第一句是从旁观者出发，此句则从病人的立场出发

Answer 3

简单通俗地讲,He should see a doctor,语气比较委婉,表示建议:应该
He must see a doctor ,语气强硬,表示命令:必须
He need see a doctor,语气适中,表示个人观点:需要

Answer 4

Should has obligation to do it

You should come to class on time. Because on time is required for everyone.

You shouldn't do that = You did it not what you should do.

Need = there is a need from either or both sides.
I need to give this book to her.

Must = have to, very strong expression of need or it can be a direct order!

You must go and carry out the job! There is no room for negotiation.

Bob