1) 영어를 할 줄 아세요? 2) 영어를 하십니까? 3) 영어 하세요? The first should be '영어 하실 줄 아세요?'. And we drop '를' a lot as the subject or object is plainly revealed during a conversation. 1) 영어 하실 줄 아세요? 2) 영어 하십니까? 3) 영어 하세요? I wouldn't say the 3th to someone older than me although there's nothing ungrammatical with it, it sounds a bit impolite. With the first, you're speaking politely and showing your interest in getting along with the listener. The second is mostly said in a formal situation, like when you're announcing your presentation in front of groups of people.