1. I agree with Phillip Mun here, -봐 basically tones down a command. Let's suppose I'm in trouble. If my friend says "말해" to me, I'll be likely to refuse to say a word about what kind of trouble I'm in. However, if she or he says "말해봐" instead, I'll feel more comfortable to talk about it. As we see, -봐 adds a certain tone that goes: "Give it a try and let's see what happens(or what we can do about it)". That may explain why I got a little irritated when an English-speaking friend said imperative sentences very often. 2. -와 means "Come here" 잡아와 = catch him or her and come here with him or her. 집어와 = pick it up and come here with it. 해와 = do it and come here with it. 3. The -는 걸 ending you're asking is one with some intonation I can't express in words, this use isn't common in daily conversation since it sounds very lyrical. 하지만 난 널 사랑하는 걸. But I love you(I can do nothing about it) 너에게 달려가고 싶지만 그럴 수가 없는 걸. I want to run to you, but there's no way of doing it. 4. -세요 makes a honorific imperative sentence. 여기 앉으세요. Please sit here. 잠시만 기다리세요. Wait a moment.

1. You can attach 봐 all verbs. 말해(tell) → 말해봐 / 열어(open) → 열어봐 / 닫아(close) → 닫아봐 / ※ 봐 would be kind of command, recommendation and sugestion. It depends on situation. If I would hear intonation, I can exactly figure out what it means. But 말해(tell) is close to command. 2. 와 is close to command than 봐. But with out 와, 봐 is more stronger. 3. I'm not sure and I'm guessing 는걸 is kind of relative pronoun. ex) Please listen to what your father is saying. We can translate -------- 아버지가 말하는 것을 들어 We can alos translate ---- 아버지가 말하는 걸 들어 4. 세요 makes words more honorific. 말해(tell) is kind of command and informal but 말하세요 is more polite 닫아(close) is kind of command and informal but 닫으세요 is more polite