The reason why they had to leave wasn't explained to us.为什么用 had to 而不用must
问题描述：
The reason why they had to leave wasn't explained to us.为什么用 had to 而不用must
答
had to - - - -指的是已经“过去”的事情,must通常是指“将来”的事情.你的句子里 wasn't 是“过去”的事情呀.这说明他们已经离开了.就必须用had to