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。