1.与现在事实相反
If+S+过去式动词,S+would(could ,should, might)+原型动词
例如:If I were rich,I would help you.
因为现在我不有钱,所以是与现在事实相反的用法
2.与过去事实相反
If+S+过去完成式,S+would(could ,should, might)+have+p.p
例如:If he had told me the truth,I would not have made the mistake.
因为已经是过去的事了,所以假设句一律用完成式
3.与未来事实相反
表示对未来事情的假设,实际上绝不可能发生.
If+S+were to +V,S+would(could, should, might)+原型动词
例如:If you were going to part with us , she would be canceling the date.
如果你今晚跟我们一起去派对, 她就要取消她的约会.
4.现在或未来可能发生的状况
If+S+现在式动词,S+ 祈使句 或 will(can,may,shall )+原型动词
例如:If it rains tomorrow,I shall not go.