He said he is busy. That is，a game of tennis made him very busy.
请老师剖析一下这句话的结构，that is 后面为何要用逗号？把 made 换成 making 可以吗？
正确的句子应该是：
He said he was busy.That is，a game of tennis made him very busy.
1. 不可以把 made 换成 making，由于 made 是谓语动词，a game of tennis 是主语，making 是非谓语动词，不可以作谓语。
2. that is = that is to say [副] （外研社、建宏《英汉多功能词典》），一般应该用逗号与句子的其它部分隔开，意思是“亦即”“那就是说”“换句话说”等。比如：
Languages are taught by the direct method, that is to say,without using students own language. 语言教学用的是直接教学法，也就是说，不需要学生的母语。
I loved him—that is, I thought I did. 我当时是爱他的，或者说，我以为我是爱他的。
（《朗文高级英语应试词典》）
He will leave Japan next Saturday, that is, June 6th. 他将在下星期六，也就是6月6日离开日本。（外研社、建宏《英汉多功能词典》）
3. 最新出版的《牛津英语使用方法指南》第三版觉得 that is to say 是一种“话语标记”，指出：“在说话人 / 作者用另一种方法将某事重说一遍时，就用 that is to say 或 in other words。如：
We cannot continue with the deal on this basis. That is to say / in other words , unless you can bring down the price we shall have to cancel the order. 大家不可以继续在这个基础上进行买卖了。也就是说/换句话说，除非你减少价格，不然大家不能不取消定货。”
