核心问题
（1）He asked him why he was doing that.
(2) That's why I've come to see you.
以上两句中的why的意义和用法相同吗？
词典解答
句(1)中的why是连接副词,引导宾语从句,而句(2)中的why是个缩合型的关系副词,即省略了中心词的关系副词,在意义和语法上相当于the reason why。
That's why I've come to see you. = That's the reason why[that] I've come to see you. 下面是此种用法的若干实例：
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That's why you need the lab. 这就是你们需要实验室的原因。
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That is why lots of people in China have healthy white teeth. 这就是为什么中国许多人都有着健康洁白的牙齿的原因。
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That's why we were getting anxious about you. 所以,我们一直为你们担心。
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That is why the Chinese people are making great efforts to protect their farmland and increase agricultural production. 这就是为什么中国人要下大气力来保护耕地，增加农业生产的原因了。
练习题
请做下面的英语试题：
- ——I drove to Zhuhai for the air show last week. ——Is that ____ you had a few days off?
A. why B. when C. what D. where(全国高考,99-13)
- I had neither a raincoat nor an umbrella. ____ I got wet through.
A. It's the reason B. That's why C. There's why D. It's how(上海高考,98-11)
- Mr. Scott cannot get a telephone for his garage. ____ he has just bought twelve pigeons. A. That's so B. That's why C. Because D. For(NCE,2,L.5)