The reason why…is that… 做…的原因是…，其中why引导的是定语从句，that引导的是表语从句，that 不可以省略。
一个句子中有两个从句，是一个非常好的高级句式。例如：The reason why he was late for school was that he got up late. 他上学迟到的原因是陵橡埋他起床晚了。
注意事项：
同学们在使用这个句型时常犯的错误是：意识不到why和that后所跟的都是从句，仅凭汉语意识写句子，不会套用句式。误把that用成because. 比如下面这些错误：
例如：The reason why was that he got up late.
The reason why he was late for school was that got up late.
The reason why he was late was because he got up late.
句型应用:
1、在感谢信中表达感谢的原因：
例如：The reason why I made so much progress was that your encouragement played an important role in my study. 我之所以取得如此大的进步，是因为你的鼓励在我的学习中发挥了重要作用。
2、在道歉信中表达道歉的原因：
The reason why I can’t go to travel with you is that I have something urgent to do next week.
我不能和你一起去旅行的原因是下周我有急事。
3、在邀请信中表达邀请的原因：
例如：The reason why I invite you to take part in the activity is that the activity can make you have a better understanding of Chinese culture. 我邀请你参加这个活动的原因尺蚂是这个活动可以使如裤你更好地了解中国文化。