英语中的用来对过去、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 “ 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”。看视频，快速学习 “第三类条件句” 的主要 。
Welcome to BBC Learning English. I’m Sam and today we’re talking about the third conditional.
The third conditional is a little strange because we’re talking about something that didn’t happen in the past. Why would we do that? Well, because sometimes we want to imagine a different outcome.
A friend of mine came to visit me yesterday. It was a surprise, so when I opened my front door, I said: "If I’d known you were coming, I would have baked a cake!"
Did I know she was coming? No. Did I bake a cake? No.
So I’m imagining a different result.
Because we’re talking about an imaginary past, we use:
If + the subject + the past perfect, the subject + would have + past participle
And you can also swap those round: What would you have done if you’d known your friend was coming?
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.
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?