Mastering 〜ことにする (Koto ni Suru): The Decisive Grammar for JLPT Success

〜ことにする (Koto ni Suru)

If you’re studying for the JLPT N3 or N2, you’ve probably encountered a dizzying array of grammar points used for talking about the future, plans, and intentions. You have 〜つもりだ (tsumori da), 〜予定だ (yotei da), and the frequently confusing pair: 〜ことにする (koto ni suru) and 〜ことになる (koto ni naru).

Before diving into the grammar details, you can always consult a complete JLPT N4 guide here: The Ultimate JLPT N4 Complete Guide: Your Gateway to Working and Living in Japan

Feeling Lost in Translation? Why Decision-Making Grammar Matters

Are you struggling to express a firm, personal decision in Japanese without sounding like you’re just making a loose plan? Do you feel uncertain about the difference between deciding something yourself and merely being informed of a decision made by others?

You’re not alone. Many learners find that while they can translate the individual words, capturing the precise nuance of commitment and personal agency in Japanese is tough.

That’s where 〜ことにする (koto ni suru) comes in. This essential grammar structure allows you to declare a decision, setting a clear boundary between your self-determined actions and external obligations. By mastering this form, you will significantly elevate your conversational Japanese and secure those critical points on the JLPT.

The Core Rules of 〜ことにする: Making a Personal Choice

At its heart, 〜ことにする expresses a decision made by the speaker (or the subject of the sentence) to perform a specific action.

What Does 〜ことにする Mean?

It translates roughly to: “to decide to do [Action],” “to make it a rule to do [Action],” or “to choose to do [Action].”

The key element here is volition—the decision is driven by the subject’s will. It shows a commitment to a course of action that was chosen from several possibilities.

How to Construct the Sentence

The usage is straightforward. You attach it directly to the plain form of a verb.

Formula:

Plain Verb Form (Dictionary Form or Negative Form) + ことにする

• Affirmative Decision: 食べる (taberu, to eat) → 食べることにする (Taberu koto ni suru) – I decide to eat.
• Negative Decision: 食べない (tabenai, not to eat) → 食べないことにする (Tabenai koto ni suru) – I decide not to eat.

Remember that like most verb phrases, you will conjugate する (suru) based on the tense:

• Present/Future Tense (Declaring the immediate decision): 勉強することにする (Benkyō suru koto ni suru) – I decide to study (now or starting now).
• Past Tense (Reporting a past decision): 勉強することにした (Benkyō suru koto ni shita) – I decided to study.

The past tense, 〜ことにした (koto ni shita), is extremely common, as you are often reporting a resolution you made earlier.

Diving Deeper: Nuance and Distinctions

While the basic function is decision-making, understanding its usage requires comparing it with related grammar points that often confuse JLPT candidates.

〜ことにする vs. 〜ことにしている: Decision vs. Habit

This is perhaps the most crucial distinction for intermediate learners.

When you use 〜ことにする, you are talking about a single, specific decision made at a particular point in time.

When you use 〜ことにしている (koto ni shite iru), you are talking about a habit, routine, or standing policy that resulted from a past decision.

Grammar Point	Function	Example
〜ことにする	Single Decision / Resolution	今日から運動することにした。(Kyō kara undō suru koto ni shita.) – I decided to start exercising from today.
〜ことにしている	Established Habit / Routine	毎日朝早く起きることにしている。(Mainichi asa hayaku okiru koto ni shite iru.) – I make it a rule/habit to wake up early every morning.

Think of 〜ことにしている as the continuous state resulting from the initial decision (〜ことにした). If you stopped exercising after a week, you would no longer use the continuous form, but you would still have the memory of the past decision (〜ことにした).

〜ことにする vs. 〜つもりだ (Tsumori Da): Commitment Level

Both koto ni suru and tsumori da express the speaker’s intentions regarding the future, but they differ significantly in the level of commitment and formality.

〜つもりだ (Tsumori da): Expresses the speaker’s intention or belief about the future. It’s a strong plan, but it lacks the official “I’ve made a firm choice” feel of koto ni suru. It’s more personal thought.

〜ことにする (Koto ni suru): Expresses a concrete, often formal or declared decision. It implies that steps have been taken, or will definitely be taken, to realize the action. It feels more definitive and structured, often signaling the end of deliberation.

• Tsumori da: 週末、映画を見るつもりだ。(Shūmatsu, eiga o miru tsumori da.) – I intend/plan to watch a movie this weekend. (Just a plan in my head.)
• Koto ni suru: 週末の映画は、ホラー映画を見ることにした。(Shūmatsu no eiga wa, horā eiga o miru koto ni shita.) – I decided on the horror movie for the weekend movie. (The choice has been made.)

When studying for the JLPT, remember this distinction: koto ni suru implies a definitive choice among alternatives.

Real-Life Examples: Applying 〜ことにする

To truly grasp this grammar, let’s look at three scenarios covering different aspects of personal and group decision-making.

Example 1: The Personal Health Resolution (Past Decision)

Imagine you have been thinking about improving your diet for weeks, and finally, you make the commitment.

日本語: 甘いものを控えめにして、野菜をたくさん食べることにしました。

Romaji: Amai mono o hikaeme ni shite, yasai o takusan taberu koto ni shimashita.

English Translation: I decided to cut back on sweets and eat a lot of vegetables.

Key Takeaway: By using ことにしました, you are communicating that this is a firm resolution you have adopted. This isn’t just a vague wish; it’s a policy you’ve set for yourself.

Example 2: The Group Decision (Negotiated Outcome)

Although koto ni suru emphasizes personal will, it is frequently used to report the collective decision of a group, such as a family, a committee, or colleagues, when the speaker is part of that group.

Imagine your family is discussing where to go for the New Year holiday:

日本語: 意見が割れましたが、結局、今年は温泉に行くことにしました。

Romaji: Iken ga waremashita ga, kekkyoku, kotoshi wa onsen ni iku koto ni shimashita.

English Translation: Opinions were divided, but in the end, we decided to go to a hot spring this year.

Key Takeaway: The phrase reports the official result of a negotiation or discussion. The action (going to a hot spring) is now confirmed as the plan based on the collective decision.

Example 3: The Negative Decision (Choosing Not to Do Something)

You can use the negative plain form (V-ない) to express a decision to refrain from an action. This is crucial for setting personal boundaries.

Imagine you are working overtime, but realize you need to protect your evenings:

日本語: 仕事が忙しいですが、もう夜10時以降は残業しないことにします。

Romaji: Shigoto ga isogashii desu ga, mō yoru jūji ikō wa zangyō shinai koto ni shimasu.

English Translation: Although work is busy, I am deciding not to do overtime after 10 PM anymore.

Key Takeaway: The decision here is active—you are actively choosing the negative action. Using ことにします makes this boundary firm and official.

Common Pitfalls and How to Avoid Them

To truly master 〜ことにする for the JLPT, you must be acutely aware of where learners often go wrong, especially regarding the difference between personal decision and external circumstance.

Mistake 1: Confusing 〜ことにする with 〜ことになる (Koto ni Naru)

This is the most frequent and costly mistake for JLPT test-takers. While they look similar (using koto and suru/naru), their meanings are diametrically opposed:

〜ことにする (Koto ni suru): You make the decision. (Active, Volitional)

〜ことになる (Koto ni naru): The decision or circumstance comes to be. (Passive, Non-Volitional)

Think of it this way:
* する (Suru) means “to do” or “to make” → You are actively making the choice.
* なる (Naru) means “to become” or “to result in” → The situation results in the outcome, often due to outside factors (rules, boss’s decision, external schedule).

Sentence	Meaning	Agency
私が東京へ引っ越すことにした。	I decided to move to Tokyo.	My choice.
私が東京へ引っ越すことになった。	It was decided that I will move to Tokyo. (I was told to move.)	External force/rule.

Pro Tip for JLPT: If the subject of the decision is not the speaker and has no apparent personal will (e.g., “The rule was decided,” “The meeting date was decided”), you must use 〜ことになる.

Mistake 2: Applying it to External Subjects

Because 〜ことにする expresses volition (will/choice), the subject of the verb must typically be capable of making a conscious decision.

You cannot generally use it for third parties unless you are reporting their expressed decision or using it in indirect speech.

Incorrect Usage Example:

X 彼女は明日、早く出かけることにした。
(X Kanojo wa ashita, hayaku dekakeru koto ni shita.)

Why is this awkward? When referring to a third person (彼女/Kanojo), Japanese usually uses expressions like 〜と言った (to itta – she said) or 〜つもりらしい (tsumori rashii – seems she intends) unless the decision is already known to be her explicit declaration.

Better (Simply reporting a third person’s decision):
○ 彼女は明日、早く出かけるそうだ。(Kanojo wa ashita, hayaku dekakeru sō da.) – I heard she is leaving early tomorrow.

Best (Reporting a group decision you are part of, but referring to the group):
○ 私たちは明日、早く出かけることにした。(Watashi-tachi wa ashita, hayaku dekakeru koto ni shita.) – We decided to leave early tomorrow.

Mistake 3: Using It for Actions Already Complete or Unchangeable Facts

〜ことにする implies an action that will be performed following the decision. You cannot use it to describe facts or things that have already happened naturally.

Incorrect Usage Example:

X 昨日の夜、雨が降ることにした。
(X Kinō no yoru, ame ga furu koto ni shita.)

Rain falling is a natural event, not a decision. Instead, you would use the simple past tense:

○ 昨日の夜、雨が降った。(Kinō no yoru, ame ga futta.) – It rained last night.

The decision (suru) must always relate to a future action that the subject controls.

Advanced Application: The Cultural Context of Decision-Making

In Japanese culture, particularly in formal or group settings, clear declaration of a decision is very important. 〜ことにする often functions as a formal closing statement to a discussion.

Imagine a lengthy meeting where everyone is debating options A, B, and C. The person chairing the meeting might conclude:

「はい、分かりました。今回はオプションBで進めることにしましょう。」

(Hai, wakarimashita. Konkai wa option B de susumeru koto ni shimashō.)

“All right, I understand. Let’s decide to proceed with Option B this time.”

The use of 〜ことにしましょう (the volitional form of the decision) is a powerful way to transition from discussion to commitment, signaling consensus and finality.

This phrasing is not just grammar; it is a vital communicative tool for confirming mutual agreement and closing the deliberation phase.

Summary: Your Path to JLPT Mastery

The grammar point 〜ことにする is a powerful tool for expressing commitment, control, and closure in Japanese. By recognizing its fundamental difference from passive statements (〜ことになる) and intentions (〜つもりだ), you can communicate with greater precision and confidence.

Review Checklist:

• Connection: Plain Verb Form (V/V-ない) + ことにする
• Meaning: A volitional decision made by the subject.
• Tense: 〜ことにした (Past Decision) or 〜ことにする (Current/Future Decision).
• Key Distinction: Use する (to do) when you are the decision-maker.

Keep practicing those crucial distinctions, especially the difference between suru and naru. Incorporate 〜ことにする into your daily resolutions—whether it’s studying 10 new kanji tonight or deciding to use more complex grammar structures in your next conversation.

Good luck with your JLPT preparation. You’ve made the decision to succeed, so now go out and do it with the Free JLPT training platform, IPPO.