这个好掌握：动词不定式的形式为：to do,to be doing,to have done.
to do表示“一般情况,或将要去做的事情”.如：He is said to be a teacher.据说他是老师.
to be doing表示“不定式的动作正在进行.如：He is said to be writing a novel.据说他正在写一本小说.
to have done表示”不定式的动作已经发生.如：He is said to have finished his work.据说他已经完成了他的工作.