If the new safety system _______ to use, the accident would never have happened.
A．had been put B．were put C．should be put D．would be put
A
【详解】
考查虚拟语气。句意：如果这个新的安全系统被投入使用过的话，这个事故就不会发生了。根据主句确定是对过去情况的虚拟，所以从句谓语用had done形式，此外根据句意可知是被动，用被动语态，故A项是正确的。
【点睛】
英语中的三种语气，语气是动词的一种形式，用来说明我们对自己的话所采取的态度。陈述语气（疑问，否定，肯定，感叹）祈使语气（命令，要求，建议，警告）虚拟语气：不可能发生的事，愿望，建议，与事实相反的假设等.虚拟语气在条件中的具体用法真实条件句If you work hard,you’ll surely pass the exam.虚拟条件句（假设情况不存在，实现机会很小）If I were a bird, I could fly freely in the sky.if从句的谓语动词主句谓语动词1.与现在事实相反动词过去式（be一律用were）would/could/might/should+动词原形2.与过去事实相反had donewould/could/might/should+have done3.与将来事实可能相反a.动词过去式（be一律用were）b.should+动词原形（不能用would）c.were to+动词原形would/could/might/should+动词原形与现在事实相反if从句：动词过去式（be---were）主句：would/could/might/should+动词原形If we had time, we would go with you.If I were you, I shouldn’t do that.If it weren’t raining, we wouldn’t stay.与过去事实相反if从句：had done主句：would/could/might/should+have doneIf the teacher hadn’t helped us, we couldn’t have succeeded.I could have done it better if I had been more careful.If he hadn’t been ill, he might have come.3.与将来事实可能相反if从句：a.动词过去式（be一律用were） b.should+动词原形（不能用would） c.were to+动词原形主句：would/could/might/should+动词原形If he came tomorrow, I would tell him everything.If it should rain tomorrow, I would stay at home and watch TV.If it were to snow tomorrow, they would not go out.Should he come tomorrow, please give the book to him.错综时间条件句在虚拟语气中，有时条件从句动作发生的时间和主句动作发生的时间是不一致的，称为错综时间条件句。这种句子的形式往往要根据句子的时间相调整，分别处理。If you had spoken to him yesterday, you would know what to do now.If you had followed the doctor’s advice then, you would be all right now.If you had watered them more, the crops would be growing still better now.(有时时间会省略）含蓄条件句有时虚拟条件不用if条件从句，而用介词或介词短语来表示。They would not be able to live without him.（如果没有）He would have failed but for his teacher’s timely advice.（要不是）=if it had not been forIf only you had worked with greater care!(要是......就好了）You would have done the same thing in her position.With a little more care you could have avoid the traffic accident.虚拟结合条件句在虚拟语气中，假设的情况有时间可以通过上下文或其他方式表现出来，并且虚拟的情况和真实的情况同时存在。I was ill that day. Otherwise, I would have taken part in the singing competition.（实------虚）=I was ill that day. If I hadn’t been ill that day, I would have taken part in the singing competition.He would have bought the piano, but he didn’t have enough money.（虚------实）=He would have bought the piano if he had had enough money, but in fact he didn’t have enough money.省略if倒装条件句在虚拟语气中，if从句假设的情况中如果有助动词had, should和were，那么可省略if，把had, should和were放在句首构成倒装，简称省略if倒装条件句。If he had finished the work yesterday, he would have gone to the concert with you.=Had he finished the work yesterday, he would have gone to the concert with you.If I were you, I wouldn’t do that.=were I you, I wouldn’t do that.If I should meet him, I would tell him about it.=Should I meet him, I would tell him about it.
过去完成时的概念：
过去完成时既可表示过去某个时间或动作之前所发生的事情，也可表示过去某个时间或动作之 前所发生的动作一直持续到过去某个时间。过去完成时由“had+过去分词”构成。
如：She had leant 2000 English words by the end of last month.
过去完成时的应用：
（1）与过去完成时连用的时间状语可以多种多样，使用它的主要依据是看其是否发生在“过去的过去”。
如：He had learnt some English before I went here. 在来这儿之前我学过一些英语。
He had written three stories by last year.到去年他已经写了三篇故事。
We had cleaned the room when he got there. 当他到那儿时，我们已经把房子弄干净了。
He went there after he had finished his work. 他把事做完之后，就去哪儿了。
注意：当主句跟由before, after, as soon as所引导的时间从句的动作连接很紧密时，从句也可用一般过去时。
如：He went there after he read the letter. 他看了那封信后就去那儿了。
（2）过去完成时还经常用于主句为过去时的宾语从句中。
如：He said he had been a soldier. 他说他当过兵。
They said they had seen the film. 他们说这幕电影他们看过。
（3）在hardly/scarcely…when…,nosooner…than…两个结构中的主句通常要用过去完成时。
如：Hardly had I told her the news when she began to cry out. 她一听到这一消息就大哭了起来。
Scarcely had he finished his supper when he went out. 他已吃完晚饭就出去了。
No sooner had he fallen asleep than he lay down on the bed. 他一躺倒床上就睡着了。
（4）动词expect, hope, suppose, think, want等的过去完成时可用来表示未实现的计划、打算或希望。
如：I had hoped to see you. 我本希望来看你。
He had wanted to buy a house in Beijing. 他本想在北京买栋房子。
过去完成时与一般过去时：
（1）过去完成时表示以过去某时间为起点以前所发生的动作或存在的状态，即：过去完成时强调“过去的过去”，而一般过去时只表示以现在时间为起点以前所发生的事情或存在的状态。
如：He studied there two years ago. 他在那儿学习了两年。(离现在两年)
He said he had studied there two years ago. 他说他在那儿学习了两年。(离他说话时两年)
（2）表示过去某个时间以前所连续发生的两个或两个以上的动作时，一般用and, then, but按照动作的先后顺序连接，此时通常用一般过去时而不用过去完成时。
如：He said he went the shop and bought some food. 他说他去商店买了一些食品。
（3）两个动作相继发生，可用一般过去时；如第一个动作需要若干时间完成，用过去完成时。
如：When I heard the news, I was very excited.
（4）叙述历史事实，可不用过去完成时，而只用一般过去时。
如：Our teacher told us that Columbus discovered America in1492.
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