So I was saying in korean "I don't speak korean well" so I've written it like this :" 한국어를 안 잘 하세요 그러나 노력해요" but apparently here I should use the verb 못해요 instead of 하세요 according to a Korean girl I'm talking with.
Could you explain me ? I'm a bit lost
Alors, she told me that 하세요 wasn't used.

Vedi una traduzione

First, 하세요 is from 하다(to do), but 하세요 is so polite (it is what we call 존댓말) that you use it to your elders or strangers, but not when talking about yourself.
Second, then a form you can use here is 해요, which is also form 하다. However, if you say 한국어를 잘 안해요, it sounds like "I don't speak Korean. (very often)"
When we say 못 instead of 안, we mean we cannot do something from lack of our ability. But when we say 안, we mean we do not do something by our will, regardless of our ability.
Many times "못" corresponds to "cannot", but I think in this case, it should be 못, because you do not mean that you do not speak Korean well by your will. Does it make sense? :)
So, correct sentence is 한국어를 잘 못해요.