以上四种类型的用法依次举例如下： 1.与现在的事实相反

If I knew his address, I would write to him.假如我知道他的地址，我就写信给他。（事实上我不知道他的地址，因而无法给他写信。）

/If I were you, I would go and tellhim the fact.如果我是你的话，我会告诉他实事。/If there were no sunlight, plants would not grow.（没有太阳，植物无法生长。）