This is the 14337th most frequent Korean word.
못하냐
"못하냐" means "Can't you do (it)?" or "Are you unable to do (it)?"
In this sentence, '못하냐' is used to inquire about the inability to perform an action, asking for a reason.
너는 왜 숙제를 못하냐?
Why can't you do the homework?
Here, '못하냐' is used in a rhetorical manner, targeting someone's skill or ability.
그가 축구를 잘 못하냐고 놀렸다.
He mocked by asking if he can't play football well.
In this case, '못하냐' is employed to ascertain the completeness of someone's inability to perform a specific job.