1、对现在的虚拟结构：if+主语+动词动词过去时，主语+should/would+动词原形 ，should不可以省略。
【例句】If I had had enough money, I would have joined the swimming club.
【翻译】如果我有足够的钱，我就能去参加游泳俱乐部了。
2 、对过去的虚拟结构：if+主语+had+过去分词，主语+should/would+have+动词过去时。
【例句】If she had told him about the danger, he would not have got hurt.
【翻译】如果她能早点告诉他危险，他就不会受伤了。
3 、对将来的虚拟结构：if+主语+should+动词原形（were to+动词原形），主语+would/should+动词原形，这个should 可以省略。
【例句】If he had followed my advice, he would not suffer so much now.
【翻译】如果他当初听我的意见，现在就不会受那么多的罪了。
虚拟条件句的从句中如果有助动词were; had或should可以去掉if以后用倒装句语序。
【例句 】：
1、Had I not helped him, he would have failed in business.
如果我没有帮助他，他就无法完成他的工作。
2、Should I be free, I would come .
如果我有空的话，我就会来。
3、Were she my daughter, I could suggest a different plan .
如果她是我的女儿,我会提出不同的计划。