与shouldn't 的含义及用法它和 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.有什么区别?

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

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

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”...

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