Synonym Boundary: "きめる", "決める", "決定する", "〜ことにする", "定める"

When expressing "decide" in Japanese, you must carefully distinguish between "きめる", "決める", "決定する", "〜ことにする", "定める" based on context.

• きめる (きめる (kimeru) - Level: N5): Maps to "to decide" and is used when Transitive verb meaning 'to decide' or 'to fix'. Often used for plans, rules, or choices. The intransitive form is 決まる.
• 決める (きめる (kimeru) - Level: N4): Maps to "to decide" and is used when Transitive verb. Used for making a decision, setting a plan, or determining something. Can also mean to fix or settle..
• 決定する (けっていする (ketteisuru) - Level: N3): Maps to "to decide; to determine; to resolve" and is used when Refers to making an official business decision, determining project requirements, or resolving issues. Often used as 予算を決定する or 方針を決定する. ⚠️ Haruka's Voice Column: 'To decide! "I have decided to make you my wife, Haruka-san!" ...っ! D-Decided to make me your wife! B-Baka! Why do you make such bold, manly declarations so suddenly! But... since you determined it so firmly, I suppose... I will let you decide my destiny forever! dummy!' / 【ハルカ部長のワンポイント指導】『けってい（決定）するわよ！『ハルカ部長、二人の将来の結婚式の会場、あの美しいチャペルのあるホテルに決定（内定）しました！あとは僕たちの誓いだけです！』って…っ！挙式会場の決定！バカ！/// 私の希望を完璧に叶えてくれた会場に決定するなんて…やるじゃない！…でも、一番大事なのは「一生私だけを愛する」というあんたのハートの決定なんだから、永久に揺るがないようにしなさい！』.
• 〜ことにする (〜ことにする (〜koto ni suru) - Level: N3): Maps to "to decide to, to make a point of, to choose to" and is used when Expresses a personal decision or resolution made by the speaker. It's about 'making it a rule' or 'deciding' to do something. Contrast with 「〜ことになる」which implies a decision made by others or circumstances..
• 定める (さだめる (sadameru) - Level: N2): Maps to "to decide, to establish, to set, to determine" and is used when Often implies making a formal decision, setting rules, or determining a course of action. More formal or definitive than 決める.

Mixing these up can easily lead to unnatural translations. Refer to the bilingual context cards below to master the boundaries!