사랑해(Sa-rang-heh), it's often said between intimate friends or couples. We say just 사랑해. or 사랑해요(Sa-rang-heh-yo, a honorific version). As for “사랑합니다(Sa-rang-ham-nee-da.”, this is what I say to my teachers and professors on Teacher's Day. Actually we rarely say 나는 당신을 사랑합니다. in our daily life. 我 refers to ‘나(I)’, 爱 ∼ ‘사랑하다(love)’, 你 ∼ ‘너, 당신(you)’, so it could be 나는 당신을 사랑합니다, but we often omit the subject or the subject and the object in spoken Korean. I will tell it's because we know who is speaking and who's listening if you wonder how it can be possible.