.I hear the headmaster is going to london next week .为什么用hear.而不用hear of 或者hear from

首先 hear of 是听说过的意思 hear from是受到某人信件的意思
此处the headmaster is going to london next week 为一个句子
为听说的内容 用法跟 say that 是一个道理··that可以省略