사랑해 i love you 사랑했어 i loved you 사랑하는 사람 beloved person

it's not really a "form" of verb the "form" would be 사랑 (love - a noun) or 사랑하다 ([사랑+ 하다] to love - a basic dictionary form), then to use it in a sentence you'd have to change it into "사랑해요" by adding a "어요/아요" ending, or "사랑합니다" by adding a "ᄇ니다/습니다" ending. You can add a whole lot of endings to it which all would have different meanings. Korean language is based on this kind of adding endings ^^; for example you mentioned 사랑하니까... it's 사랑하다 + 니까 ending, and the 니까 ending gives it the meaning "I love (you/him/her/something) so..." I can't explain properly so I won't add it as an answer just comment ^^ hope it helps!