〜ことにする: Deciding To

English packs a whole mental event into one small verb: decide. Japanese refuses to hide the machinery. To say "I decided to run every morning," it builds 毎朝走ることにする — literally "I make it into the matter/fact that I run every morning." The nominalizer こと turns the action run every morning into a thing, and する ("do / make") is you reaching out and choosing it. The decision is not reported from the outside; it is enacted, and the する puts your will on the table. This is the agentive pole of the whole intention-and-change system — the deliberate opposite of 〜ことになる, where a decision simply comes about without your hand on it.

The form

Take a verb in its plain form — dictionary form for a positive decision, plain negative for a "decide not to" — and attach ことにする:

The こと here is the abstract nominalizer — the same one covered under formal nouns こと・もの・の. It converts an entire clause into a noun so that する can take it as its object.

Present ことにする vs. past ことにした

The tense of する is not a small detail — it changes what speech act you are performing.

• 〜ことにする (present): you are making the decision now, in front of the listener. It reads like a resolution or an on-the-spot choice: "right, I'll do X."
• 〜ことにした (past): the deciding is already done. You are announcing a decision you have reached — the English "I've decided to…".

Register note: 〜ことにした is the plain (informal) form for friends and diary-style writing; 〜ことにしました is its polite (formal) counterpart for coworkers, teachers, and anyone you'd use ます with. The construction itself is register-neutral — only the ending on する moves.

Deciding not to: 〜ないことにする

To decide against an action, negate the inner verb, not する. The verb goes into its plain negative (〜ない form) and ことにする follows:

This is where English-speaker instinct goes wrong. English negates the verb decide ("I decided not to go"), so learners reach for ×行くことにしなかった. But 行くことにしなかった would mean "I didn't make the decision to go" — a report that no deciding happened, not a decision against going. The negation belongs on the action: 行かないことにした = "I decided not to go."

〜ことにしている: a rule you've made for yourself

Put する into its 〜ている (stative) form and the meaning shifts from a single decision to a standing decision you keep — a self-imposed habit or personal rule. English reaches for "I make it a rule to…," "I make a point of…," or "I always…".

The logic is transparent once you see it: a single decision is する; a decision you keep holding is している. The 〜ている is the same continuous-state marker you already know — here it says the decision is still in force. Note the crucial contrast with 〜ようになる: 歩くことにしている is a habit you consciously imposed on yourself, while 歩くようになった would say the habit simply developed in you. Volition vs. spontaneous change — the axis this whole subgroup turns on.

The idiom hiding inside: 〜たことにする / 〜なかったことにする

Because ことにする literally means "treat it as the fact that…," Japanese uses it for a very human move: pretending. Attach it to a past-tense clause and you get "let's decide it did happen" or, far more commonly, "let's decide it didn't happen" — i.e. act as if.

Why こと + する at all? The volitional pole

Step back and the structure explains itself. する is the verb of doing and making. Give it an action-turned-noun (こと) and に (the particle of into / to a resulting state), and you get "I make [this action] into [my chosen course]." The subject is unmistakably the agent: you performed the act of deciding. That is why every ことにする sentence carries a faint fingerprint of personal responsibility — you own the choice.

Hold this against its twin, 〜ことになる. Swap する for なる ("become") and the agency drains out: 転勤することになった = "it came to be that I'll transfer" — the company decided, circumstances closed in, and you are merely reporting the outcome. The entire decision-and-change grid — laid out on the する / なる comparison page — pivots on exactly this one swap. Anchor ことにする firmly as the する / I-act member, and its opposite number will make immediate sense.

Common mistakes

Use ことにする only when the decision is genuinely yours. If circumstances or other people decided, Japanese expects the humbler 〜ことになる.

You cannot drop こと. する needs a noun to "make into a decision," and こと is what converts the verb into that noun.

Negate the action, not する. This is the single most common error transferred straight from English word order.

The particle is に because you turn the action into (に) a decided course, not because you "do the action of running." Memorize ことにする as a fixed unit.

Don't confuse ことにする (a discrete decision) with 〜ようにする (a sustained effort). やめることにした is a done deal; やめるようにした is you working at it.

Key takeaways

• ことにする = "I decide to" — literally "I make it the fact that…," with する marking the decision as your act of will.
• ことにした (past) is the everyday "I've decided to…"; ことにする (present) announces a decision at the moment of speaking.
• Negate the inner verb — 〜ないことにする = "decide not to" — never the する.
• ことにしている = a decided-on habit / personal rule ("I make it a rule to…").
• The contrast with ことになる is the whole game: する = I chose it; なる = it came about.