if 的虚拟语气和省略形式

时态	从句谓语动词	主句谓语动词
现在	\(\text{did/were}\)	\(\text{would/coud/should/might + do}\)
过去	\(\text{had done}\)	\(\text{would/coud/should/might + have done}\)
将来	\(\text{did/were, were to do, should do}\)	\(\text{would/coud/should/might + do}\)

省略形式：当从句谓语动词为 \(\text{were, had done, should do}\) 时，可以去掉 \(\text{if}\)，并把 \(\text{were, had, should}\) 提前。