This is the 7836th most frequent Korean word.
행복하다
To be happy.
In this sentence, '행복한' is used as an adjective in its stems form to describe 'day.'
오늘은 정말 행복한 하루였어요.
Today was truly a happy day.
Here, '행복하게' is the adverbial form, modifying the verb '살다' to express the manner of living.
행복하게 살고 싶은 사람은 자기 꿈을 따라가야 해요.
People who want to live happily should follow their dreams.
In this sentence, '행복하다' is used in its plain present form within an quoted clause to convey a statement of happiness.