Deciding to do... (~koto ni suru)

1. 1こと ({koto}) - The Nominalizer: The word こと is a versatile noun that can mean "thing," "matter," or "fact." In grammar, one of its most powerful functions is as a nominalizer. It takes an entire preceding verb clause and packages it into a single noun concept. For example, the action 海外で働く(kaigai de hataraku) ("to work abroad") becomes 海外で働くこと(kaigai de hataraku koto) ("the act/matter of working abroad"). This transformation is essential because you can't make a decision on a verb; you must first turn the action into a conceptual noun that can be chosen.

1. 1に ({ni}) - The Particle of Selection: The particle に has many functions, but here it acts as a marker of selection or determination. Think of how you order food: コーヒー(ko-hi-)にする ({ni suru}) means "I'll have the coffee," where に pinpoints "coffee" as your choice from a list of options. In our grammar pattern, に targets the entire nominalized こと phrase as the object of your decision. You are, in effect, pointing to "the act of working abroad" and selecting it.

1. 1する ({suru}) - The Verb of Action and Decision: する means "to do," but in this construction, its meaning is closer to "to make it so" or "to opt for." As a highly volitional verb, する requires an agent—a conscious doer. When you combine it with the [action] + こと + に structure, する becomes the engine of the decision. It's the final click that says, "This is what I am choosing to do."

They are very similar and often interchangeable. 決める ({kimeru}, "to decide") tends to carry a stronger sense of finality, resolve, and conscious deliberation. する ({suru}) is more general and can be used for everything from major life choices to minor, spontaneous ones. For a big decision like quitting a job, you could use either 辞める(yameru)ことにした or 辞める(yameru)ことに決めた. The 決めた version adds a slight emphasis on the firmness of the resolve.

This is a key distinction between a decided action and a mere intention.

• ~しようと思う ({shiyou to omou}): "I think I will..." This expresses an intention or thought. It's softer, less certain, and suggests the plan is still forming in your mind. 来年、日本に行こうと思う(rainen, nihon ni ikou to omou) (I'm thinking of going to Japan next year.)
• ~ことにする: "I have decided to..." This expresses a concluded decision. The deliberation is over, and the choice is made. 来年、日本に行くことにした(rainen, nihon ni iku koto ni shita) (I've decided to go to Japan next year. - The tickets might already be booked).

Not at all. The base form is neutral/plain, and you control the politeness by conjugating the final する ({suru}). This makes the pattern incredibly flexible.

• Casual: ~ことにする / ~ことにした
• Polite (General): ~ことにします / ~ことにしました
• Humble (Your own action): ~ことにいたします / ~ことにいたしました (Used in formal business settings)
• Honorific (Someone else's action): ~ことになさいます / ~ことになさいました (Used when speaking respectfully about a superior's decision)

Only if you are choosing a simple noun. For example, コーヒー(ko-hi-)にします ("I'll have coffee") is perfect. However, if your decision involves a verb/action, you must use こと to nominalize the verb clause first. You cannot say ❌ 食べるにします ({taberu ni shimasu}). It must be 食べること(taberu koto)にします.