如何判断英语中虚拟语气是对现在还是什么动作的假设,比如说如果我是你,这是对什么时候的假设,我越来...

如何判断英语中虚拟语气是对现在还是什么动作的假设,比如说如果我是你,这是对什么时候的假设,我越来...
如何判断英语中虚拟语气是对现在还是什么动作的假设,比如说如果我是你,这是对什么时候的假设,我越来越搞不明白了,

首先回答你的例子,假如我是你,应该这么说 If I were you,这表示与现在事实想反的假设 ,用过去时,遵守如下第1条的第（1）个规则：
1、虚拟语气用于条件状语从句中
（1）表示与现在事实相反的假设,条件状语从句中的谓语动词用“过去式（be动词的过去式用were）”,而主句中的谓语动词用“would / should/ could / might + 动词原形”.如：
If I were a boy, I would join the army.
If the had time, she should go with you.
（2）表示与过去的事实相反,条件状语从句中的谓语动词用过去完成时,主句中的谓语动词则用“would / should / might / could + have +过去分词”.如：
If he had taken my advice, he would have succeeded in the competition.
（3）表示与将来事实相反,条件状语从句中的谓语动词一般过去时或should(were to) + 动词原形,而主句中的谓语动词则用would / should/
could might + 动词原形.如；
If it were to rain tomorrow, the football match would be put off.
（4）当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时,动词的形式要根据它所表示的时间作相应调整.如：
If they had worked hard, they would be very tired.(从句说的是过去,主句指的是现在)
以下表格是虚拟语气用于条件状语从句中时,主句和从句谓语动词的形式：
条件状语从句主 句
与过去事实相反had + 过去分词should /would/could/might + have + 过去分词
与现在事实相反一般过去时(be用were)would/should/could/might + 动词原形
与将来事实相反一般过去时或should(were to) + 动词原形Would/should/cold/might + 动词原形
有时侯在使用时可省略if,句子则可换成下列形式,即“were / had / should +主语”.如：
Were I a boy, I would join the army.
Had he taken my advice, he would have succeeded.
Were it not for the expense, I would go to Britain.
2、虚拟语气用于名词性从句
（1）虚拟语气在宾语从句中的运用.
①“wish + 宾语从句”表示不能实现的愿望,译为“要是……就好了”等.表示现在不能实现的愿望,从句中的谓语动词用一般过去时；表示将来不能实现的愿望,从句中的谓语动词用“would/could + 动词原形”；表示过去不能实现的愿望,从句中的谓语动词用“had + 过去分词”或“could(should) + have + 过去分词”.如：
I wish it were spring all the year round.
I wish I had known the answer.
I wish I could fly like a bird.
②在表示建议、要求、命令等的动词suggest、advise、propose、demand、require、insist request、command、order等后的宾语从句中,谓语动词用should + 动词原形或是动词原形.如：
She suggested we (should)leave here at once.
The doctor ordered she should be operated.
（2）虚拟语气在同位语从句和表语从句中的运用.
作表示建议、要求、命令等的名词advise、idea、order、demand、plan、proposal、suggestion、request等的表语从句和同位语从句,从句中的谓语动词用“（should） + 动词原形”.如：
His suggestion that we (should)go to Shanghai is wonderful.
My idea is that they (should)pay 100 dollars.
（3）虚拟语气在主语从句中的运用.
在主语从句中,谓语动词的虚拟语气用“should + 动词原形”的结构,表示惊奇、不相信、理应如此等.如：
It is necessary(important, natural, strange, etc.)that we should clean the room every day.
It was a pity (a shame, no wonder, etc.)that you should be so careless.
It will be desired(suggested, decided, ordered, requested, proposed, etc.)that she should finish her homework this afternoon.
注意：这种从句表示的是事实.如果说人对这种事实表现出惊奇的情感,就可用虚拟语气.反之,如果不表示惊奇等情感,that从句也可用陈述句语气.如：
It is pity that you can’t swim.
3、虚拟语气在其他场合的运用
（1）虚拟语气在as if/as though、even if/even though等引导的表语从句或状语从句中,如果从句表示的动作发生在过去,用过去完成时；指现在状况,则用一般过去时；指将来状况则用过去将来时.如：
He did it as if he were an expert.
Even if she were here, she could not solve the problem.
（2）虚拟语气用于定语从句中.
这种从句常用于句型“It is (high)time (that) … ”中,定语从句的谓语动词用一般过去时(be用were)或should + 动词原形,意思是“(现在)该……”.如：
It’s time that I picked up my daughter.
It’s high time we were going.
（3）虚拟语气用在if only引导的感叹句中.如：
If only I were a bird.
If only I had taken his advice.
（4）虚拟语气在一些简单句中的运用.
①情态动词的过去式用于现在时态时,表示说话人谦虚、客气、有礼貌或语气委婉,常出现在日常会话中.如：
It would be better for you not to stay up too late.
Would you be kind enough to close the door?
②用于一些习惯表达法中.如：
Would you like a cup of tea?
I would rather not tell you.