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”。
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?
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?
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?
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?
