ㄷ can be pronounced as ㄴ, ㄸ, ㅈ 1) 문 닫는다 [문 단는다] ⇒ You can figure out why it's like that if you think how 't' is pronounced in "Look at Nice(a city in France)" 2) 닫다 [닫따] ⇒ㄷ+ㄷ=ㄸ 3) 굳이 [구지] ⇒ ? It's called "palatalization", you have to memorize them - they are few. ㅂ can be pronounced as ㅁ, ㅂ, ㅃ 1) 밥 먹어 [밤 머거] ⇒ "Let me see" 2) 국밥 [국빱] If one syllable ending with ㄱ / ㄷ / ㅂ / ㅅ / ㅈ / ㅊ / ㅌ / ㅍ combines with another syllable ending with one of those aforementioned, the latter one is doubled.

ㄷ: when it come in the front of the word its pronounced as T,and when it come between the letters of the word its pronounced as D. ㄱ: when it come in in the front and in the end of the word it pronounced as K,and when it come between the letters of the words it pronounced as G. ㅂ:Mainly its B ,but when it come it the end "sometimes not all the word!" its pronounced as M for ex.입니다=>"입",고맙습니다=>"습". ㄹ: its L,but when it come between two vowels its pronounced as R for ex.바래. thats all :) hope was helpful :) and plz if any one see thats my answer wrong correct it !