"항상 (그리고) 영원히" hangsang (geurigo) yeongwonhi As you know, Translating a song is not that simple. It may be affected in many cases by rhythm and rhyme. Of course the tempo of the song as well. So i'd better let you know each elements you've shown. My baby : 나의 사랑(naui sarang) I love you : 1.사랑해(saranghae) / 2.사랑해요(saranghaeyo) / 3.사랑합니다(saranghamnida) 1: Intimate, friendly 2: Natural, kindly, respectful 3: Formal, elegant and : 그리고(geurigo) I'll miss you : 1. 보고 싶을거야(bogo sipeulgeoya) / 보고 싶을거예요(bogo sipeulgeoyeyo) 2. 그리울거야(geuriulgeoya) / 그리워 할게요(geuriwo halgeyo)

I assume 언제까지나 is the most proper one, it includes the meaning of the words both. As I'm looking into it, it's more often used than anything else in lyrics.

always and forever 언제나 그리고 영원히 I just wanna little edit his answer. 항상 is not wrong, but that is not that good to that expresion. 언제나 is better.