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含义不同、用法不同、侧重点不同。that's because后接某事发生的原因；that's why后接某事产生的结果。that is why的含义是这就是为什么、因此；that is because的含义是那是因为、那是…的原因、这是因为。
that用法
1.that作指示代词
That is my book.那是我的书。
2.that作指示形容词
We haven't found any solution to deal with that problem yet.我们仍未找到解决问题的办法。
3.that作关系代词引导定语从句
This is the sort of thing that Jeremy loves.这是杰里米喜欢的东西。
4.that作连词
that可以作连词引导各种名词性从句，包括宾语从句、主语从句、同位语从句和表语从句。
1)that作连词引导宾语从句
They have recommended that the site be closed and repaired. 他们建议将该处关闭并进行维修。
2)that作连词引导主语从句
That you will win the medal seems unlikely. 你想获得奖牌看起来是不可能的。
3)that作连词引导同位语从句
The idea that modern art can only be seen in museums is mistaken. 现代艺术只能在博物馆才能见到的想法是错误的。