As you know, some letters can take the form of two different sounds. When a letter has a different pronunciation depends on whether the letter is used at the beginning or ending of a syllable. It can get a lil tricky sometimes. 멋있다 would be pronunced as "meo-shi-ta" The ㅅ after 머 takes the place of the silent ㅇ. So, 멋있 becomes 머 싰 The double consonant ㅆ is at the end of the syllable so it sounds like "t". Normally ㄷ sounds like "d" but you wouldn't say "meo-shi-TDa" so it sounds like, meo-shi-ta. You'll soon learn that Koreans like to pronunce things the easy way :) Hope that helps.

Something like this: 멋있다 = meo-shit-da 멋있어요 = meo-shi-seo-yo 멋있었어요 = meo-shi-seo-seo-yo