Isn't 'yamero' extremely rude? Or does the 'desu' at the end cancel that out?
It's the "ro" ending that makes it brusque. "Yameru" is completely neutral, it just means to quit.
Which is why you shouldn't start in the first place.
きんえんする means "abstaining from smoking" rather than quitting, so it wouldn't be OK here.
Can someone explain me the のは in this sentence? How do you use?
Good question. Earlier we were taught by Duo that the の, added to something, adds the "ing" ending. [Or so I thought I understood...] That would make it "quitting smoking" rather than "quit smoking."