英语中的条件句用来对过去、现在或将来发生的事情或情况作出假设。在对过去的情景作出假设、假想时，应该使用 “第三类条件句（the third conditional）”。这类句子的典型结构是：“If + 主语 + 过去完成时，主语 + would have + 过去分词”。

用法总结

1 句式

典型的第三类条件句的句式如下：

If + 主语 + 过去完成时，主语 + would have + 过去分词

这个句式的前半部分（If + 主语 + 过去完成时）被称作 “条件从句” 或 “if 从句”；后半部分（主语 + would have + 过去分词）被称作 “主句”。条件从句（if 从句）和主句之间的顺序可以互换，意思不变。主句在前时，主句和从句之间不使用逗号。

If I'd known you were coming, I would have baked a cake.

I would have baked a cake if I'd known you were coming.

2 用法

第三类条件句被用来对过去不曾发生的事情作设想。条件从句（if 从句）对过去作设想，主句讲述设想的结果。

If I'd been born in Spain, I would have learned Spanish, not English!

What would you have done if you'd failed your test?