Koto ni Suru and Koto ni Naru: I Decided vs It Was Decided

Japanese has two ways to announce a decision, and choosing between them says as much about who decided as it does about what happens next.

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What you need to know

Koto ni Suru and Koto ni Naru: I Decided vs It Was Decided

~Koto ni suru means to decide to do something, with the speaker as the one making the choice. ~Koto ni naru means it has been decided, or it has come about, and it keeps the decision-maker in the background. That difference matters in conversation, because Japanese speakers often prefer koto ni naru when announcing news at work, even about their own choices, to sound modest and less self-assertive. The same pair has two useful extensions: koto ni shite iru for personal rules and habits you keep, and koto ni natte iru for rules, policies and arrangements that already exist. This guide covers the formation of all four patterns, the nuance of each, and how to use them in everyday speech and at work.

Koto ni suru: I decided to, a choice the speaker makes

Koto ni naru: it has been decided, by circumstance or someone else

Koto ni shite iru describes a personal habit or rule you keep to

Koto ni natte iru describes an existing rule, policy or arrangement

Two ways to talk about decisions

Both patterns use koto, the nominaliser that turns a clause into a thing. Nihon ni iku koto is going to Japan as an idea. Ni suru means to make it into, and ni naru means to become. So iku koto ni suru is literally to make going into the plan, and iku koto ni naru is going becomes the plan.

That literal difference maps neatly onto meaning. With suru, a person acts and chooses. With naru, the outcome arrives, and the speaker does not say who decided. This is similar to the general contrast in Japanese between transitive and intransitive views of events.

JapaneseRomajiMeaning
来年、日本に行くことにしました。Rainen, Nihon ni iku koto ni shimashita.I've decided to go to Japan next year.
来年、日本に行くことになりました。Rainen, Nihon ni iku koto ni narimashita.It's been decided that I'll go to Japan next year.
今日はタクシーで帰ることにする。Kyō wa takushī de kaeru koto ni suru.I'll take a taxi home today. (deciding now, casual)
来月から大阪で働くことになった。Raigetsu kara Ōsaka de hataraku koto ni natta.It turns out I'm going to work in Osaka from next month. (casual)

How to form all four patterns

All four follow the plain dictionary form or the plain nai form of a verb. Nouns take ni suru and ni naru directly, without koto: kōhī ni shimasu, I'll have coffee, is the same idea of choosing, applied to a noun.

Tense matters. Koto ni suru in the present often means deciding right now, as in kore ni suru, I'll go with this. Koto ni shita, in the past, reports a decision already made. Koto ni naru in the present can describe what will happen as a result, while koto ni natta announces news.

PatternExampleMeaningNuance
～ことにする行くことにするI'll decide to goDeciding now
～ことにした行くことにしましたI've decided to goDecision made by me
～ないことにした行かないことにしましたI've decided not to goNegative decision
～ことになった行くことになりましたIt's been decided I'll goOutcome, decider unstated
～ことにしている毎朝走ることにしていますI make a point of running every morningPersonal rule or habit
～ことになっている九時に始まることになっていますIt's set to start at nineExisting rule or arrangement

Koto ni suru: owning your decision

Use koto ni shimashita when you want to make it clear that the choice was yours. It sounds decisive and is common when talking to friends about plans, resolutions and changes of heart.

The negative form, ~nai koto ni suru, is just as common: amai mono o tabenai koto ni shita, I've decided not to eat sweets. It often pairs with a reason given with n desu or node.

Compared with tsumori desu, which states an intention, koto ni shimashita emphasises the moment of deciding. Iku tsumori desu is I intend to go; iku koto ni shimashita is I've made up my mind to go.

JapaneseRomajiMeaning
いろいろ考えて、会社を辞めることにしました。Iroiro kangaete, kaisha o yameru koto ni shimashita.After a lot of thought, I've decided to quit my job.
今年はお酒を飲まないことにしたんです。Kotoshi wa osake o nomanai koto ni shita n desu.I've decided not to drink this year.
雨だから、今日は家にいることにする。Ame da kara, kyō wa ie ni iru koto ni suru.It's raining, so I'm going to stay in today.
じゃあ、この案で進めることにしましょう。Jā, kono an de susumeru koto ni shimashō.Right, let's go ahead with this plan.
聞かなかったことにします。Kikanakatta koto ni shimasu.I'll pretend I didn't hear that.

The last line shows a special use: past tense plus koto ni suru means to treat something as if it were so. Mita koto ni suru, to count it as seen, and nakatta koto ni suru, to act as if it never happened, are common idioms.

Koto ni naru: news, outcomes and modesty

Use koto ni narimashita to announce results, especially those decided by a company, a school or circumstances: transfers, schedules, events and official changes. The listener understands that the speaker is reporting an outcome.

It is also the classic modest form for personal news. Many people announce marriages, moves and resignations with koto ni narimashita, even though they chose them, because presenting the news as something that came about sounds humble and avoids putting yourself at the centre.

In the present tense, koto ni naru also describes consequences: kono mama da to, maniawanai koto ni naru, at this rate, we will end up missing the deadline.

JapaneseRomajiMeaning
このたび、結婚することになりました。Konotabi, kekkon suru koto ni narimashita.I'm happy to announce that I'm getting married.
四月から福岡支社に転勤することになりました。Shigatsu kara Fukuoka shisha ni tenkin suru koto ni narimashita.I'm being transferred to the Fukuoka branch from April.
会議は来週に延期されることになりました。Kaigi wa raishū ni enki sareru koto ni narimashita.The meeting has been postponed until next week.
このままだと、終電に間に合わないことになるよ。Kono mama da to, shūden ni maniawanai koto ni naru yo.At this rate we'll miss the last train.
大変なことになった。Taihen na koto ni natta.This has turned into a real mess.

Koto ni shite iru: habits you keep

Koto ni shite iru is the te iru form of koto ni suru. It describes the ongoing result of a decision: I have decided, and I keep to it. That makes it ideal for personal rules, routines and health habits.

It is more deliberate than a plain habit. Mainichi arukimasu, I walk every day, simply reports a routine. Mainichi aruku koto ni shite imasu, I make a point of walking every day, says you chose this rule and follow it on purpose.

JapaneseRomajiMeaning
毎日、一駅歩くことにしています。Mainichi, hitoeki aruku koto ni shite imasu.I make a point of walking one station's distance every day.
夜十時以降はスマホを見ないことにしています。Yoru jūji ikō wa sumaho o minai koto ni shite imasu.I don't look at my phone after 10 p.m. as a rule.
寝る前に必ず日本語を声に出して読むことにしている。Neru mae ni kanarazu Nihongo o koe ni dashite yomu koto ni shite iru.I always read Japanese aloud before bed. (casual)
仕事の話は家でしないことにしているんです。Shigoto no hanashi wa ie de shinai koto ni shite iru n desu.I make it a rule not to talk about work at home.

This pattern is also a polite way to decline. Osake wa nomanai koto ni shite iru n desu, I make it a rule not to drink, sounds like a personal principle rather than a rejection of the person offering.

Koto ni natte iru: rules and arrangements

Koto ni natte iru describes something that has been decided and is now in place: rules, policies, customs and scheduled arrangements. You will hear it constantly from staff, landlords, teachers and colleagues explaining how things work.

It sounds softer and less personal than a direct prohibition. Instead of saying something is banned, the speaker explains that this is simply how things are set up, which removes any sense of personal criticism.

JapaneseRomajiMeaning
ゴミは朝八時までに出すことになっています。Gomi wa asa hachiji made ni dasu koto ni natte imasu.Rubbish is supposed to be put out by 8 a.m.
こちらの部屋はペット不可ということになっています。Kochira no heya wa petto fuka to iu koto ni natte imasu.Pets aren't allowed in this apartment.
チェックインは三時からとなっております。Chekkuin wa sanji kara to natte orimasu.Check-in is from three o'clock. (very polite)
会議の資料は前日に送ることになっている。Kaigi no shiryō wa zenjitsu ni okuru koto ni natte iru.We're supposed to send meeting materials the day before.
明日、田中さんと会うことになっています。Ashita, Tanaka-san to au koto ni natte imasu.I'm scheduled to meet Mr or Ms Tanaka tomorrow.

Choosing between the four

When in doubt, ask two questions: who made the decision, and is it a one-off or ongoing? That usually tells you which of the four fits.

Who decided?One-off decisionOngoing state
Me, and I want to show it～ことにしました～ことにしています
Others, circumstances, or I want to sound modest～ことになりました～ことになっています
  • Telling a friend you've decided to learn to cook: ryōri o narau koto ni shita.
  • Announcing a transfer at work: tenkin suru koto ni narimashita.
  • Explaining your daily jog: maiasa hashiru koto ni shite imasu.
  • Explaining a building rule: yoru wa sōon ni ki o tsukeru koto ni natte imasu.

Common learner mistakes

  • Using the masu form before koto: ikimasu koto ni shimashita is wrong; use iku koto ni shimashita.
  • Adding koto after nouns: kōhī koto ni shimasu is wrong; say kōhī ni shimasu.
  • Using koto ni shimashita for company decisions, which sounds as if you personally made the call.
  • Confusing koto ni shite iru with a plain habit; it implies a deliberate choice, so it can sound odd for involuntary routines like waking up hungry.
  • Mixing up koto ni natte iru and koto ni naru: the first is an existing rule, the second is a new outcome or consequence.
Key points

A practical way to improve

Say two decisions you made this week with koto ni shimashita, then announce one piece of news as if at work with koto ni narimashita. Add one personal habit with koto ni shite imasu and one rule at your home or workplace with koto ni natte imasu. Then ask your AI Sensei in Unihongo's immersive 3D classroom to run the Speech at a Farewell Party mission, and announce that you are moving on using koto ni narimashita. Finish with the View an Apartment mission and ask about house rules, listening for koto ni natte imasu in the answers.

  1. 1

    Choose one clear goal

    Focus each session on a situation or skill you can actually use.

  2. 2

    Practice actively

    Say complete answers aloud instead of only reading or recognizing Japanese.

  3. 3

    Review and repeat

    Keep useful corrections and revisit the same skill until it feels natural.

FAQ

Frequently asked questions

What is the difference between koto ni suru and koto ni naru?

Koto ni suru emphasises that the speaker chose: tenshoku suru koto ni shimashita, I've decided to change jobs. Koto ni naru presents the outcome as decided by circumstances or others: tenkin suru koto ni narimashita, I'm being transferred, or it has been arranged that I move.

Why do Japanese people use koto ni narimashita for their own decisions?

It sounds more modest and less self-centred, as if the result emerged from circumstances rather than personal will. It is very common in formal announcements such as leaving a company or getting married, where drawing attention to your own decision could feel too assertive.

Is koto ni shite iru the same as itsumo?

Not quite. Itsumo simply says always. Koto ni shite iru says you have made it a rule for yourself and keep to it deliberately: mainichi aruku koto ni shite imasu, I make a point of walking every day.

How do I say something is against the rules in Japanese?

Use the negative with koto ni natte iru: koko de wa tabako o suwanai koto ni natte imasu, smoking is not allowed here. It is softer than kinshi desu, prohibited, and is common when staff explain rules politely.

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