would 和should的区别,什么时候用would?什么时候用should?

虚拟语气的表达形式是通过动词的变化形式表达的， 其特点是主从句时态的不一致， 而且一般有明显得标志。具体如下：
一：if 句型 (共有三种句型)
1. 与现在的事实相反：
从句用一般过去时，主句的谓语用would (could, might) ＋动词原形
If I were Bill Gates, I would not work so hard every day.
2. 与过去的事实相反：
从句用过去完成时，主句的谓语用would (could, might) ＋ 现在完成时
If I had gone to America when I graduated from middle school, I would have got my PhD degree.
3. 与将来的事实相反：
从句用should (were to，did) + 动词原形，主句的谓语用would (could, might) ＋动词原形
If it should/were to snow tomorrow, I would go skiing.
4. 虚拟语气条件句中if的省略，此时主谓要倒装。
Had he worked harder, he would have got through the exam.
5. 条件句发生的动作和主句发生的动作不一致，(例如：条件句动作发生在过去，主句的动作发生在现在)谓语动词要根据表示的时间进行调整。
If you had taken the medicine yesterday, you would feel much better now.
If I were you, I wouldn’t have missed the film last night.
二： wish 句型 表达“但愿…，要是…多好”的语气
wish后宾语从句用虚拟语气，其谓语部分有时态变化：表示对现在情况珠虚拟时，动词用一般过去时(如did)；对过去的情况进行虚拟时，动词用“过去完成时(如had done)或情态动词的过去时 + 动词的现在完成时(如could have done)”形式；表示情况的虚拟时，用“情态动词的过去时形式 + 动词原形(如might do)。”
I wish I had been to the concert last night.
I wish he would forgive me.
I wish I would remember all the English words in a week.
同例： 在as if /as though引导的状语从句中和以 if only引起的感叹句中，谓语动词与wish引导的宾语从句中的虚拟形式相同。例：
He talked as if he had known Tom for a long time.
He talks as if he were the boss. 他说起话来就像他是老板。
If only I were free now. (注意： if only…后面可以不加主句 )
三：在强制性语气的宾语从句中， 即表示建议、命令、劝告、决心等主观色彩的动词 + that + (should) + 动词原形， 其中should 经常被省略。这类动词包括：
suggest, propose, recommend, demand, order, command, desire, require, insist, advise, decree (发布命令；下令), determine, prefer, stipulate, move(动议，规定)，direct(命令)，maintain(坚持)，decide，ask
I suggest that you (should) not be late again next time.
I prefer that you ( should ) not do that. 我认为你还是别干那件事的好。
注意一： 以上动词转化为名词(如：suggestion, proposal, demand, order, command, advice, desire, requirement, request, agreement, determination, preference , resolution, indication 等)后接同位语从句或表语从句时，从句要求用虚拟语气，其谓语部分用“(should)+ 动词原形” (should可省略)。
We are all for your proposal that the discussion be put off.
His demand is that all of us (should) be present at the meeting.
注意二：以上这类动词中有时候有的动词的宾语从句不用虚拟语气，必须根据句子的意思来判断。
What he said suggested(表明)that he did not agree with us.
三：It is/was + 形容词 / 过去分词 + that + (should) + 动词原形，这些形容词主要表示必要性、重要性、强制性、合适性、义务性，即某人对某事的反应。
important, natural, desirable, possible, astonishing, advisable(可取的， 明智的), anxious, appropriate, compulsory(义务的；强制的；强迫的), crucial (至关紧要的), eager, essential, fitting, imperative (命令的， 强制的， 必要的), improper, natural, necessary, obligatory (义不容辞的， 必须的), preferable, proper, urgent, vital, willing shocked, requested
It was essential that the application forms be sent back before the deadline.
It is requested that a vote be taken. 有人提请投票表决。
It is necessary that we all should do our best to protect environment around us.
注意：表示不可思议、滑稽、不可想象、令人吃惊这样的形容词如amazing, strange, odd, ridiculous, surprising, unthinkable，incredible等后的that从句中should一般不省略，而且翻译为“竟然”， 表示说话人的惊异、懊悔、失望等情感。
It is surprising that they should pass the time like that.
It is incredible that Jane should have finished her paper so soon.
It is strange that there should be any hope of finding the lost child.
四：在 would rather…， would sooner…， had rather…， would just as soon…， would prefer… 意为“宁可，但愿”。从句用虚拟语气，若表示现在或将来的事情，谓语用过去时；若表示过去的动作，用过去完成时。
I would rather that you painted the room green.
I’d just as soon you had returned the book yesterday.
五：It’s (about/high/ good ) time that…，表示“该是。。。的时候了”， 含有 “晚一点” 的意思， 表示建议现在应该做什么事，从句一般用一般过去时。
It’s time you went to bed.
It’s high time that we took action.
六：在lest that …， for fear that…， in case that … 引导的表示消极意义的目的主语从句中常用虚拟语气，表示“唯恐，以免”，从句用 should +动词原形。
He put his coat over the child for fear (lest )that he should catch cold.
He emphasized it again and again lest she (should) forget.
Take a hat with you in case the sun is very hot (注意：该句陈述某一事实)
七：含蓄虚拟条件句
含蓄虚拟条件句是指没有出现由if引导的条件句，而条件句的意思是用其他方式表达的。 如：without, but for, otherwise，or, but that, given, provided, supposing, were it not for等等。
Without your help, we couldn’t have finished the work on time.
But for the English examination I would have gone to the concert last night.
I wouldn't have succeeded without your help.
We didn’t know his telephone number; otherwise we would have telephoned him

right.

第一人称时表自己主观意愿时多用should,其他人称时,表示他“应当”那样时,多用should,其他表示非主观意愿,以及客观要求时,多用would