always; without fail; certainly
Expresses strong certainty or obligation: something will definitely happen or must be done. Often used in rules, promises, and predictions.
Please be sure to submit your homework.
He always keeps his promises.
I will definitely keep my promise.
Please be sure to come at nine o'clock tomorrow.
When spring comes, the cherry blossoms are sure to bloom.
必ず is an adverb meaning 'without fail', focusing on inevitability, while 間違いない is an adjective-like expression stressing certainty of truth.
必ず stresses certainty or inevitability ('without exception'), while 是非是非 stresses the speaker's strong personal desire or invitation.