题目内容：

关于一般现在时和现在完成时
我在黑皮英语高一书上看到：
用一般现在时代替完成时,当谓语动词为hear ,tell ,learn ,know 等时,可以代替.如I hear (have heard )he will go to London .
但是此句中：Both his parents look sad .Maybe they have known what's happened to him .句中have known 又不能被know 代替
请问为什么不能?上述的代替是否有什麽条件?