表虚拟语气

If they were here,they would help you.
if从句时态后退，主句谓语用“would/could/might/should+动词原型。

I wish I could fly.

我希望我能飞。我其实没能力飞，所以用虚拟，时态退后一步用过去时。

上网搜吧，虚拟语气的讲解多着呢。如果有不明白的我愿意帮忙