how可引导哪些从句,只说从句名称即可

楼上正解,不过说得有点太缩略了.
how不能引导定语从句和状语从句,但是可以用来引导名词性从句,即所谓的宾语从句/主语从句/表语从句/同位语从句.
如：I don't know how I can get to the post office.宾语从句
The question is how you can finish it in one hour.表语从句
How it happened is still a secret.主语从句