forever indebted

Translations
навсегда в долгу
He is forever indebted to her for their kindness.
Он навсегда в долгу перед ней за их доброту.
Definitions
A phrase indicating a deep sense of gratitude or obligation to someone for a significant favor or help received, suggesting that the speaker feels they can never fully repay the kindness.
Words
indebted

adjective
обязанный