In terms of grammar, "마요" is correct but "말아요 is often used more than "마요".^^ I also prefer to use "말아요" rather than "마요". @jon 웃지 마십시오 - formal and polite 웃지 마세요 - informal and polite 웃지 마요 - informal more and polite 웃지 마 - informal and impolite ^_______^