Korean Grammar Notes TTMIK
Book 2: lesson 18

To be good/poor at ... / 잘하다, 못하다

[object] + -을/를 (object marking particle) + 잘하다 = to do [object] well; to be good at [object]

[object] + -을/를 (object marking particle) + 못하다 = to do [object] poorly; to be bad at [object]