赖世雄教你学英语语法
虚拟语气
5-2：if的省略
虚拟语气的if从句中，若有过去完成时助动词had，或表“万一”的助动词sbould或是were出现时，可将这些词类置于主语前，而将if省略。
1.had
例：if he had done it, he would have felt sorry.
= had he done it, he would have felt sorry.
（如果他当时做了这件事，他会后悔的。）
2.should
例：if he should tell lies, i would punish him.
= should he tel lies, i would punish him.
（要是他说谎，我会处罚他。）
3.were
例：if he were lazy, he might fail.
= were he lazy, he might fail.
（如果他怠惰，他就可能会失败。）