This is the 1248th most frequent Korean word.
행복한
Happy
'행복한' is used here as an adjective describing the noun '마음', expressing the state of happiness the subject feels.
저는 행복한 마음으로 하루를 시작했어요.
I started my day with a happy heart.
'행복한' modifies '미소' to describe the smile that conveys happiness.
그녀의 행복한 미소가 모두를 기쁘게 했어요.
Her happy smile made everyone cheerful.
'행복한' qualifies the noun '삶', suggesting the nature of life being full of happiness.