고마워요 is correct spelling. 고맙다 is a ㅂ irregular verb. According to vowel harmony rule, 고마와요 is correct. But most Korean people say 고마워요 instead of 고마와요 for easy pronunciation. So 고마워요 was accepted as a correct spelling. ㅂirregular verb/adjective rule is: When a word stem with final consonant ㅂ + a particle start with vowel, ㅂ changed 우 or 오. * ㅂ => 오 (only before 아 vowel) => 와 There are only two 돕다, 곱다 in this category. 돕(다) + 아요 => 도와요 곱(다) + 아요 => 고와요 돕(다) + 으면 => 도우면 곱(다) + 으면 => 고우면 * ㅂ => 우 어렵다, 쉽다, 덥다, 춥다, 귀엽다, 입다, 아름답다, 맵다, 눕다, 줍다,반갑다,사납다.... 덥(다) + 어요 => 더워요 반갑(다) + 어요 => 반가워요 덥(다) + 으면 => 더우면 반갑(다) + 으면 => 반가우면 NB. 입다, 잡다, 씹다, 좁다, 넓다, 뽑다, 접다 are not ㅂirregular verb/adjective