きっと・必ず・絶対に
surely; without fail; absolutely (never)
きっと + guess · 必ず + habit / promise · 絶対(に) + strong claim
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Three words of certainty, each with its own job.
きっと is a confident guess about something you don't control: 明日はきっと晴れるでしょう, "I'm sure it'll be sunny tomorrow". It pairs with だろう and と思う.
必ず means "without fail", for habits, rules and firm promises: 着いたら必ず電話して, "be sure to call when you arrive".
絶対(に) is the strongest and most emotional, and it's the only one that works with a negative: 絶対に誰にも言わないで, "don't ever tell anyone".
The classic trap: 必ず can't be used for a guess about the weather or someone else's result. That's きっと.
Examples
明日はきっと晴れるでしょう。
I'm sure it'll be sunny tomorrow.
着いたら必ず電話してください。
Be sure to call me when you arrive.
このことは絶対に誰にも言わないで。
Don't ever tell anyone about this.
きっと合格するよ。
I'm sure you'll pass.
毎朝必ずコーヒーを飲む。
I have a coffee every morning without fail.