关于虚拟语气的特殊情况----------高手帮忙
4、通过连词（otherwise︱or︱or else否则）、（but︱but that）、once︱though等表达条件
（1）She came to town yesterday,otherwise︱or I would not have met her.
昨天她到城里来了,不然我就不会遇见她了.
（2）I would have attended the meeting,but I have been too busy.
（3）Einstein cared little for money,though he could have been very rich.
爱因斯坦对钱不感兴趣,尽管他本来可以成为非常富有的人.
（4）Once lost,it would be hard to find.一旦失去,便不容易找到了.
（5）We should have come,but that we had an accident.倘若无事故,我们早就来了.
我想知道,这些连词引导的从句都是虚拟语气,而主句,有的是虚拟语气,有的是真实的,怎么回事?