1. if 从句表达的意思和现在的情况相反或目前不可能实现
If I had time, I would go now.
If 条件句：did, 主句：would+do
值得注意的是，be动词在虚拟语气中必须用were, 不论其原来 的形式是is , am 还是are.
If I were a bird, I would fly.
2. if 从句所表达的意见与过去事实相反
例句1：If I had married you, you would have been the president.
If 条件句：had done, 主句：would+have done
例句2：:If china had not implanted the policy of “reform and opening to the outside”,in would not have been so prosperous.
If I should/were to win a lottery of 10 million dollars, I would buy you a big house.
If 条件句：should/were to 主句： would+do
在if 条件句中含有should, had, were 时，常常会把if省去，并将shoud,had, were 放在句首(其余部分顺序不变)，这样就构成了文法上的”倒装虚拟句”。这种句子出了形式不同外，和正常的虚拟句的功能是完全一样的。
例句1： If I were you, I would go.
倒装虚拟句： Were I you, I would go.
例句2：If I should win a lottery, I would take a vacation in Europe.
倒装虚拟句：Should I win a lottery, I would take a vacation in Europe.
例句3：If you had come here earlier, we would have finished the job now.
倒装虚拟句：Had you come here earlier, we would have finished the job now.
例题1：If there had been a power failure, the hospital will run on electricity from its own generator, which can operate for 200 hours.
(A) If there had been
(B) If there were
(C) Had there been
(D) Should there be
(E) There had been
例题2： He was convinced that if property taxes would have rose any higher, he would have had
to move a different area.
(A) if property taxes would have rose
(B) if property taxes would rise
(C) if property taxes had risen
(D) had property taxes rose
(E) were property taxes to rise
分析： 句中用的是would have had的形式，那么if条件句中的对应形式应该是过去完成时的形式，所以选择C选项。
例题3： By painting them this afternoon, the walls would be completely dry by tomorrow evening.
(A) By painting them
(B) If they would have been painted
(C) Were they to be painted
(D) After painting them
(E) They would be painted.