The rule is like this: When following a full morpheme(noun, adjective, adverb, verb etc) start with ‘ㅏ, ㅓ, ㅗ, ㅜ, ㅟ’ vowel after a final consonant(받침), the final consonant changed into representative sound and became a first sound of next syllable. ex) 밭 아래[바다래] 늪 앞[느밥] 젖어미[저더미] 맛없다[마덥따] 겉옷[거돋] 헛웃음[허두슴] 꽃 위[꼬뒤] exceptionally, 맛있다, 멋있다 can be pronounced as [마싣따], [머싣따] 맛있다, 멋있다 should be pronounced as [마딛따], [머딛따] as a rule. But, many Korean people pronounced those as [마싣따], [머싣따] in conversation. So 맛있다, 멋있다 have double standards of pronunciation.