在新概念三24课中有这么一句话：But George was unsympathetic. ‘ Oh, that,' he said with a smile as if he were talking about an old friend. ‘ That's Sebastian. You forget that I was a medical student once upon a time.'请问这里为什么不用过去完成时 as if he had been talking about an old friend.以表示对过去的虚拟，因为这里是与过去事实相反。

在外国小说中看到这么一句话Tomorrow will be Christmas...... Jim. Don’t look at me that way. I had my hair cut off and sold it because I couldn’t have lived through Christmas without giving you a present. Jim, it will grow quickly. You don’t mind, do you ? 请问这里couldn't have done是表示对过去的虚拟吗？可以改为couldn't do吗？表示如果没有给你礼物，我将无法度过圣诞节，对现在或者将来的虚拟。