How do you say "ON" in English?

When someone does you a favor, you feel that you owe him "on"(窶ｰﾂｶ).
窶喇f you don't pay him back, you don't know on(窶ｰﾂｶ窶冦窶堙ｧ窶堋ｸ).

you could say it in a number of ways, like to "owe a debt of gratitude" or to "owe someone a favor." A common phrase is to say "I owe you one," when someone does you a favor. It can be translated as obligation, but that's a stiff way to say it, and infers it's a kind of duty.

I can't think of any phrase like 恩知らず, just to say they don't pay back their debts