1st irregularity---some korean characters are simplified and may sound like these:
ㅋ,ㄲ will sound like ㄱ
ㅈ, ㅉ,ㅊ, ㅅ, ㅆ will sound like ㄷ before a consonant or after a word
ㅃ, ㅍ will sound like ㅂ
ex: 있는 --> pronounced ee-neun as opposed to eet-neun.
2nd irregularity---changing the sound of a few characters if it comes before certain other characters
ㅁ, ㄴ (nasal sound)- make words flow better. if any of "throat" sounds occur before these two consonants, it will change into ㅇ sound.
ex: 학년 --> pronounced hangnyeon as opposed to haknyeon
먹네 --> pronounced meongne as opposed to meokne.
ㅂ, ㅃ, ㅍ will change into ㅁ sound before ㅁ or ㄴ (makes the flow better)
ex: 합니다 --> pronounced hamnida as opposed to hapnida.
ㅅ, ㅆ, ㅈ, ㅉ, ㅊ, ㄷ, ㄸ - will change to ㄴ sound before an ㅁ or ㄴ
ex: 꽃미남 --> pronounced kkonminan as opposed to kkochminam.