expression, godan verb (-u)
vow to oneself; promise oneself; make a silent vow
Used when someone makes a firm internal resolution, often without speaking it aloud. Common in narrative and emotional contexts.
彼は二度と嘘をつかないと心に誓った。
He vowed to himself never to lie again.
私は成功するまで諦めないと
I have promised myself not to give up until I succeed.