당신을 사랑합니다 – Dangshin uhl saranghabnida means I love you
‘당신을’ can be omitted, since Koreans usually do not use the word ‘you.’
The following all mean “I love you” (without the ‘you’ pronoun) and are listed in order of decreasing formality.
사랑합니다 – saranghabnida ( modest form for I love you)
사랑해요 – saranghaeyo (general form for I love you in Korean)
사랑해 – saranghae (used most often to boyfriends, girlfriends and such)