if引导的从句用过去式还是用过去完成时呢

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虚拟语气的形式和用法
1、对现在的虚拟，表示与现在事实相反的假设时，条件状语从句的谓语动词用过去式（be的过去式用were.而主句中的谓语动词用would (should,could, might) + 动词原形。见下表：
条件状语从句的动词形式
主句的动词形式
If+主语+ 动词过去式 (be的过去式were.在口语等非正式场合中，I，he，she，it等后面也可用was.但在If I were you中，一定要用were，不能用was.
I (we)should+动词原形
主语 + would (might, could)十动词原形

例句：
·If I were you, I should study English.(fact: I am not you, so I shall not study English.)
·If he had time, he would attend themeeting. (fact: He does not have time, so he will not attend the meeting.)
·If they didn’t take physical exercisesevery day, they wouldn’t be so healthy. (fact: They take physical exercisesevery day, so they are very healthy.)
·If you went to bed earlier, you wouldnot be so sleepy in the morning. (fact: You often go to bed late, so you arealways sleepy in the morning.)
·If this were the case, it would be veryawkward. (fact: This is not the case, soit is not awkward.)

2、对过去的虚拟，表示与过去的事实相反的假设时，条件状语从句的谓语动词用过去完成时, 主句的谓语动词用would (should,could, might)+过去分词。见下表：

条件状语从司的动词形式
主句的动词形式
If+主语+had+过去分词
I (we)should+have+过去分词
主语+could (might,could)+have ＋过去分词

例句：
·If you had taken my advice, you wouldn’thave failed in the examination. (fact:You did not take my advice, so you have failed in the examination.)
·I would have checked my paper again if Ihad had more time at yesterday’s examination. (fact: I did not have more time at yesterday’s exam, so I didn’t checkmy paper again.)
·He would have already recovered from hisillness if he had seen the doctor in time. (fact: He didn’t see the doctor intime, so he has not recovered from his illness.)
·If we had known that she was to arriveyesterday, we could have met her at the station. (fact: We didn’t know that shewas to arrive yesterday, so we could not meet her at the station.)
·You didn’t let me drive. If we haddriven in turn, you wouldn’t have got so tired. (fact: We didn’t drive in turn,so you have got so tired.)
3、表示与将来事实相反的假设，对将来表示怀疑，或将来的动作不太可能实现时，条件状语从句和主句的谓语动词有以下形式：（见表）
条件状语从句的动词形式(三种)
主句的动词形式
(1) If + 主语 + 动词过去式 (be的过去式用were). 通常要与一个表示将来的时间状语连用. (2)If+主语(任何人称和数)+should+动词原形
(3)if+主语(任何人称和数)+were to+动词原形
I (we)should+动词原形
主语+would (might, could)+动词原形

例句：
·If it rained tomorrow, our picnic wouldbe put off.
·If it should rain tomorrow, our picnicwould be put off.
·If it were to rain tomorrow, our picnicwould be put off.
·If you should fail, what would you do?
·If I were to see her tomorrow, I wouldtell her the truth.
·If it were to rain tomorrow, the meetingwould be put off.
·I bet Brazil will win the World Cup. Ifit should lose, I would treat you a meal.
注意：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来调整。
If you had worked hard, you would be very tired. (从句说的是过去，主句指的是现在。)
If he had followed the doctor’s advice, he would bequite all right now.
If I were you, I would have gone home.
Ifshe hadn’t trained so hard, she wouldn’t be able to run so fast.
虚拟语气的其他句型和用法
1、用在“wish+宾语从句”中，表示很难实现或不能实现的愿望，可以翻译为“但愿…”, “悔不该…”等。表示现在不能实现的愿望，从句的谓语动词用过去式；表示将来的愿望，从句的谓语动词用“would(could)+动词原形”；对过去的事情表示愿望，从句的谓语动词用“had+过去分词” 或“could + have + 过去分词”。Wish 后面的宾语从句必须用虚拟语气，不管这个愿望是否能实现。如：
I wish it were spring all the year round.
I wish you could go with us.
We wish we had arrived there two hours earlier.
注意：wish 与hope接宾语从句的区别在于：hope表示一般可以实现的希望，宾语从句用陈述语气。Wish表示很难或不大可能实现的愿望，宾语从句用虚拟语气。请比较：
We hope we will succeed. (We don’t know if we will succeed.)
We wish we would succeed. (We know we can hardlysucceed.)
2、用在as if/as though, 或even if/eventhough引导的从句中: 这时，如果从句中表示的动作发生在过去，则谓语动词用had＋过去分词形式；如果指的时现在的状况，则用过去式 (be用were)；指将来则用would(should,could)+动词原形。这里只看从句动作发生的时间，与主句的时态没有关系。
Forexample:
Some metals look at first sight as though they weregold.
He looked as if he were an artist.
He speaks English so fluently as if he had studiedEnglish in the U.S.
He always sounds as if he would help you.
When a pencil is partly in a glass of water, itlooks as if it were broken.
Even if he were here, he could not solve theproblem.
3、在It is (high)time (that)…句型中，后面的从句的谓语动词常用过去式 (be的过去式用were), 或用should+ 动词原形 (should 不能省略) 来表示。这句话的意思是“（现在）该…”。For example:
It is time we left/should leave.
It is high time that you got married.
It is time that I bought a car.
Now it is time that you made some sentences by usingthis pattern. Think about what you should do now.
4、在 I would rather(that)…句型中，后面的从句的谓语动词用过去式 (be的过去式用were)。这句话的意思是“我宁可…”，表示说话人的意愿。
Forexample:
I’d rather I were in the rain now.
I’d rather you did not tell him.
5、在表示建议、要求、命令等意思的动词，如arrange,command, demand, desire, insist, order, propose, request, require, suggest,beg, move（提议）后面的宾语从句中，谓语动词用should+ 动词原形，should可以省略，而引导宾语从句的that 不能省略。例如：
We suggested that we (should) have a meeting.
We insisted that they (should) go with us.
The doctor ordered that she (should) stay in bed fora few days.
He demanded that we (should) start right away.
The commander ordered that the bridge (should) bebombed.
The villagers requested that the old building not bepulled down.
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