Duolingo. (Just hit a 365-day streak, tyvm!)
I put in も—
あなたを永遠にも愛します。
Duo wants 永遠にあなたを愛します, but is も so wrong, when we mean forever? Even until forever.
Why we're using a masu form for someone we'll love forever is beyond me, but, hey; so is を, which tags you as the direct object of my loving. あなたのこと?
(My soft heart loves this song, of a pair of songs, which uses that construction in the context of love:)