Quoted from
http://www.italki.com/question/259476
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.
People commonly used "맛이 있다" instead of "맛있다".
맛이 있다[마시읻따] can be heard as [마싣따] in conversation.
When people say "맛있다", they pronounce it as [마싣따] habitually.
So 맛있다, 멋있다 have double standards of pronunciation.