that's why +句子（句意为这件事的结果）
that's because+句子（句意为这件事的原因）
比如我们要表达“她没有考过的原因是没有复习”,在两个不同的上下文可译为：
That's why she didn't pass the exam.（这时的that就指“她没有复习”）
That's because she didn't review.（这是的that就指“她没有考过”）
也可译为：
The reason why she didn't pass the exam is that he didn't review.