It's because that we only pronounce 'ㄱ, ㄴ, ㄷ, ㄹ, ㅁ, ㅂ, ㅇ' as pat'chim. According to the rules, 'ㄲ' , 'ㅋ','ㄳ' are pronounced as "ㄱ" 'ㅅ, ㅆ, ㅈ, ㅊ, ㅌ' ⇒ 'ㄷ' 'ㅍ' ⇒ 'ㅂ' 'ㄵ' ⇒ 'ㄴ' 'ㄼ', 'ㄽ', 'ㄾ' ⇒ㄹ 'ㅄ' ⇒ ㅂ like the phrase you put as an example, "맛있어요" is pronounced as "마딛써요" 1.맛 ⇒ 맏 (because pat'chim ㅅ is pronounced as ㄷ) 2.있 ⇒ 읻 (ㅆ is pronounced as ㄷ ) 3.맏읻 ⇒ 마딛 (a final consonant can be assimilated to following vowel) 4.마딛어요 ⇒ 마딛써요(ma-dit-sseo-yo) (after pat'chim 'ㄱ','ㄷ','ㅂ', following syllables sound stronger.^^) The same goes for "맛없어요" 1.맛 ⇒ 맏 2.없 ⇒ 업 3.맏업 ⇒ 마덥 4.마덥어요 ⇒ 마덥써요.(ma-deop-sseo-yo) It looks pretty complicated, but you don't need to memorize all these rules. Just try to pronounce each word exactly and speed up a little and then see if what happens. The similar things happen in English. for example, A cup of tea ə-kəp-əv-ti: ⇒ ə-kə-pəv-ti: ⇒ ə-kə-pə-ti: \^o^/

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안녕하세요 from what my Korean friend said, madeopseoyo sounds more naturally than maseopseoyo ^^