肯定 vs 确定 vs 一定 (sure / certain)

Three shades of being sure

• All hover around "sure / certain" — but split by job.
• to be certain肯定kěn dìng = definitely / for sure (a confident assertion).
• definite确定què dìng = to confirm / be certain (a verb: to make or be sure).
• definitely一定yí dìng = certainly / must (an adverb of certainty or insistence).

to be certain肯定kěn dìng / definitely一定yí dìng = "definitely"

• he / him他tādefinitely一定yí dìngcan / will会huìto come来lái = "he'll certainly come."

he他tācertainly一定yí dìngwill会huìcome来lái。

• he / him他tādefinitely一定yí dìngcan / will会huìto come来lái = "he'll certainly come."

definite确定què dìng = to confirm / be certain

• I / me我wǒstill / also还háinot / haven't没méidefinite确定què dìng = "I haven't confirmed (it) yet."

I我wǒyet还háinot没méiconfirm确定què dìng。

• I / me我wǒstill / also还háinot / haven't没méidefinite确定què dìng = "I'm not sure yet / haven't decided."

Recap

Read the full reference: HSK 4 grammar — Comparing similar functions.