Bob could have seen the famous star _yesterday morning.A did he come B if he came C had he come
问题描述：
Bob could have seen the famous star _yesterday morning.A did he come B if he came C had he come
D if he should come
答
这一题的答案应该是 if he had come.
这一题旨在于考查虚拟条件句
整个句子讨论的是过去的情况,但是这一情况与事实不符（BOb昨天上午若是来了的话,他会看到那位著名影/歌星的.事实是Bob昨天上午没有去,他也没有看到那个“星”）故使用了虚拟条件句.
主句用了情态动词+不定式的完成时表示过去本能够做什么事但实际上却未做,条件从句用退后时态法使用过去完成时构造虚拟语态表示这件事之前实际并未做.
Pattern：if sb.had done sth.,sb could have done sth..