have to 后因该加什么?

have to do
have sb do
have sth. done

动词原型。不得不做什么的意思
不是特殊情况（have 和 to是两个结构，而不是have to短语）就这一种用法。

have to
不得不做什么
后面加动原

have to +v

have to do 必须做某事...
have sb do 让某人作某事...