VOA一分鐘英語：第三類條件句用法總結

英語中的條件句用來對過去、現在或將來發生的事情或情況作出假設。在對過去的情景作出假設、假想時，應該使用 “第三類條件句（the third conditional）”。這類句子的典型結構是：“If + 主語 + 過去完成時，主語 + would have + 過去分詞”。
 
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?
 
用法總結
1 句式
典型的第三類條件句的句式如下：
If + 主語 + 過去完成時，主語 + would have + 過去分詞
這個句式的前半部分（If + 主語 + 過去完成時）被稱作 “條件從句” 或 “if 從句”；后半部分（主語 + would have + 過去分詞）被稱作 “主句”。條件從句（if 從句）和主句之間的順序可以互換，意思不變。主句在前時，主句和從句之間不使用逗號。
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
第三類條件句被用來對過去不曾發生的事情作設想。條件從句（if 從句）對過去作設想，主句講述設想的結果。
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?