He must have had an accident,or he       then.

A.would have been here

B.had to be here

C.should be here

D.would be here

must have done表示对过去事件进行很肯定的推断，即他肯定出车祸了。or意为“否则的话，要是他没有出车祸的话”。这里隐含了一个虚拟条件句。因此后半句他那个时候就该在这里了。（事实上他那个时候并没有在这里。）因此后半句是虚拟条件句的主句。then表示过去时间“那个时候”，主句表示和过去事实相反，谓语应该为情态动词的过去形式+have+done。因此A为正确答案。

B的错误在于虚拟条件句的主句谓语必须包含情态动词的过去形式。C和D的错误在于不定式用do的形式表示和现在或将来的事实相反。而then明显提示过去时间。

                              含蓄虚拟条件句

含蓄虚拟条件句: 在表示假设的虚拟语气中，有时假设的条件并不以条件从句的形式出现，而是通过上下文或句中内在的逻辑关系，或用其它形式表达出来，这就是“含蓄条件句”(或者说，有时句子没有直接给出假设情况的条件，而是用介词或连词通过上下文或其他方式来表示虚拟.比如介词短语常用来表示虚拟条件的介词有without, with, in, under, but for，under…condition that从句等)，他们表示的虚拟条件都可以被if从句替换。

常用的有以下四种：

有时虚拟条件不用从句，而是通过上下文或用介词短语等表示出来，这种句子叫含蓄虚拟条件句。常用的介词有with，without，but for要不是。等。

A:暗示在各种短语中

(1)条件暗含在without的介词短语中

  He would not get such a result without your help.

=He would not get such a result if he had not got your help (现在----过去)

没有你的帮助，他就不会有今天的成果。

【条件暗含在介词短语without your help中】

【从句说明“过去” ，主句说明“现在” 即从句表示对“过去事实虚拟和假设”；主句表示对现在的事实虚拟和假设”】

Without electricity life would be quite different today.

=If there were no electricty , life would be quite different

=If there were not  electricty , life would be quite different day

如果没有电，今天的生活就会完全不同。

【条件暗含在介词短语without electricity中】

What would I have done without you?

=What would I I have done if I didn’t have you

=What would I have done if I were not with you.

=What would I have done if I were without you.

如没有你，我会怎么办呢？

【条件暗含在介词短语without you中】

【备注：without, with ;without表示否定的条件，意为if…not；with与without意义相反，表示肯定的条件。】

Without air (= If there were no air) , there would be no living things.

如果没有空气，就不会有生物。

With his help (= If I had his help), I would do the experiment well.

如果有他的帮助，我会做好这个实验。

Without your instruction I would not have made such great progress.

=If I had not got your instruction, I would not have made such great progress.

要是没有你的指导，我不会取得如此大的进步。

(2)条件暗含在but for短语中. (如果没有,要不是)，表不在某种条件下会发生/产生某种结果。

(but for sth=if it were not for sth; if it had not been for sth=对过去事实虚拟假设)

But for their help (= If it were not for their help), we could not get over the difficulties.

要不是他们的帮助，我们是克服不了这些困难的。

But for your help, we wouldn’t have finished the task.

But for his help we would be working now.

要不是他的帮助，我们还会在工作呢。

But for the storm, I would have arrived earlier.

=If it hadn’t been for the storm, we would have arrived earlier.

   But for you, I could not be recovered so soon.

要不是有你，我就不会恢复得这么快。

【条件暗含在But for you中】

But for your help，we couldn’t have succeeded.

要不是你的帮助，我们是不会成功的

【条件暗含在But for your help中】

But for your help we couldn’t have succeeded in the experiment.

如果没有你的帮助，我们的实验是不会成功的。

【条件暗含在But for your help中】

(3)条件暗含在but that短语中. (如果没有,要不是)，表示不在某种条件下会发生某种结果

But that (若非)you helped us, we wouldn’t have finished the task.

If it were not that you helped us , we would not have finished the task

若不是/如果你不帮助我们，我们就不会完成这项任务。

But that you had seen me in the water, I would have drowned.

要不是你看见我掉在水里，我早就被淹死了。

备注：but也可以表示暗示条件虚拟句

I should have returned the book last week, but I was so busy that I forgot to do so.

我本来应该在上周归还那本书的，但我太忙了，把这件事给忘了。

I would have come to see you but I was too busy.

我本该来看你了，然而我太忙了。(come---came-come; run-ran-run)

(4)条件暗含在happening in the past短语中

  This same thing, happening in the past, would lead to a disaster.　

=This same thing, if it had happened in the past, it would lead to a disaster.

同样的事，如发生在过去，就会酿成大祸。

【条件暗含在分词短语happening in the past中】

This same thing, happening in wartime, amount to disaster.

同样的事，如发生在战时，就会酿成大祸。

【条件暗含在分词短语happening in wartime 中】

(5)条件暗含在or/otherwise短语中表示“否则，如果不”

  He must have been there, or he never could know the place so well.　

改成条件虚拟句：If he had not been there, he never could know the place so well.

他一定是去过那儿，否则他绝不会对那个地方如此熟悉。

He must have the strength of a hippopotamus, or he never could have vanquished that great beas
他一定是力大如河马，否则他绝不会击败那只庞大的野兽。

【暗含条件是连词or】

She wasn’t feeling well，otherwise she wouldn’t have left the meeting so early.

她感到不舒服，不然她不会那么早离开会场的。

【暗含条件是连词otherwise】

I was ill that day. Otherwise I would have attended the meeting

He felt very tired yesterday, or he would have attended the party.

他昨天很累，不然他就参加那个聚会了。

He was having a meeting, otherwise he would have come over to help us.

他当时正在开会，否则他就会来帮助我们了。