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」。看視頻，快速學習 「第三類條件句」 的主要用法。
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」；後半部分（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.
- 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?