certain
adj.adj. feeling completely sure about something or knowing that it is true. You use this when you have no doubts at all.
adj. possessing a firm conviction or total lack of doubt regarding a fact or outcome. Often used predicatively with 'that' clauses or 'of' phrases to indicate subjective confidence.
I am certain that I locked the door.
The manager was certain of the team's success after seeing their final practice session.
While the evidence appeared strong, the lead investigator was not yet certain enough to make a formal accusation without further forensic confirmation.
From Middle English certeyn, certein, certain, borrowed from Old French certain, from a Vulgar Latin unattested form *certānus, extended form of Latin certus (“fixed, resolved, certain”), of the same origin as cretus, past participle of cernere (“to separate, perceive, decide”). Displaced native Middle English wis, iwis (“certain, sure”) (from Old English ġewiss (“certain, sure”)) and alternative Middle English spelling sertane (“some, certain”).
When used before a noun, it can also mean 'specific but unnamed' (e.g., 'a certain person').
I am certain for the resultI am certain of the resultThe adjective 'certain' takes the preposition 'of' or 'about', not 'for'.