Yeah, He is right but If you want to make it more exactly, You can say, "당신을 절대 잊지 않을 거에요" Dang-shi-n(eu)l Jeol-d(a)e It-jji A-n(eu)l Geo-E-yo. (u) (u)

"잊지 않을 거에요" ItJi AnHeol Jeo-E-Yo