Formal:
* Je t'aimerai pour toujours. (This is the most literal translation and sounds quite poetic)
* Je t'aimerai jusqu'à la fin de mes jours. (This translates to "I will love you until the end of my days.")
Informal:
* Je t'aimerai toute ma vie. (This is a more common way to say "I will love you all my life.")
* Je t'aimerai toujours et à jamais. (This translates to "I will always and forever love you.")
Romantic/Passionate:
* Je t'aimerai éternellement. (This translates to "I will love you eternally.")
* Mon amour pour toi est éternel. (This translates to "My love for you is eternal.")
Remember:
* "Je t'aime" means "I love you" in French and is a very common expression of love.
* The choice of pronoun (tu or vous) depends on the level of formality and the relationship between the speakers.
* The verb "aimer" can also be used in other tenses, such as "j'ai aimé" (I loved) or "j'aimerais" (I would love).
No matter which phrase you choose, make sure to say it with sincerity and passion!