4. 는걸 Neun gul
It's a verb ending. I've just checked your Korean level, shin. There is no need for you to learn it yet, it's what intermediate or advanced Korean learners learn. ;)
1. I thought you(or we) are my frined.
Chin-goo = friend
Seng-Gak-ka-da = think
2. Don't do it(to your friend same age or younger than you)
Ha-dah(basic form of the verb) = do
3. Up-dah(basic form of the verb) = there's no / do not exist
I have no money Na Do-nup-dda (between close friends same age or younger than you)
I have no money Ju Do-nup-ssu-yo (to elders)
A : Hey. Do you have any extra money?
B : No?
A : Come on, I have a hot date today! Why don't you lend me some money?
B : I said I DO NOT have money! Na Do-nup-dda-Go!