expression, ichidan verb
be ungrateful; forget a kindness
Used when someone fails to appreciate or repay a favor or kindness received. Often carries a moral or reproachful tone.
彼は助けてもらったのに、恩を忘れてしまった。
Even though he received help, he forgot the kindness.
恩を忘れるような人間にはなりたく
I don't want to become a person who forgets favors.