[英语语法手册]虚拟语气 C
虚拟语气和情态动词
虚拟结构(不论从句或主句)常可用情态动词的过去式，即could，might，wou1d等加不带to的动词不定式或不带to的动词不定式完成式，来作谓语动词。这些情态动词除表示虚拟结构外，本身还有独立的意思。如：
If I were you，I Would go.我如果是你，我就愿意去。
If he were here，he might agree with you.假如他在这里，他可能会同意你。
I Would help you if I could. 假如我能够的话，我一定帮助你。
If it had not been for their help，we could not have succeeded. 如果没有他们的帮助，我们是不可能成功的。