Particles #2: Use にする to Express a Decision or Change
Updated: Apr 13, 2021
The sentence pattern "私は [noun] にする" has a similar meaning as "I've decided on [noun]" in English.
The construction "私は [object] を [adjective] (に)する" indicates that the speaker made some kind of change happen (as denoted by the adjective) to the object.
Sentence Pattern #1
(私は) [Noun] にする。
(Watashi wa) noun ni suru.
(I) have decided on [noun]
In English, we sometimes use the verb "do" to indicate a decision. For example, "We're doing dinner and a movie tonight" really means "We've decided to go out for dinner and a movie tonight." However, there are not many proper expressions like this, and it sounds strange in most cases (e.g., "I going to do basketball after school.")
In this regard, the verb "to do" (i.e., する) is much more versatile in Japanese than in English. It can be used to express deciding on almost anything.
Ocha ni suru.
I'll have tea. / I've decided on tea.
iPhone 12 o kau koto ni suru.
I'm going to buy an iPhone 12. / I've decided to buy an iPhone 12.
(If you're unsure how the above example works, check out my notes on forming sentences with a verb phrase as the topic.)
Sentence Structure #2
(私は) [object] を [na adjective] にする。
(Watashi wa) [object] o [na adjective] ni suru.
(I) made [object] [na adjective].
(私は) [object] を [i adjective] する。
(Watashi wa) [object] o [i adjective] suru.
(I) made [object] [i adjective].
When にする comes after an adjective instead of a noun, it no longer implies that a decision was made. Instead, it implies that someone or something has instigated some change as denoted by the adjective.
When the sentence has a na adjective, you need to put に between the adjective and する. You do not need to add に for i adjectives—however, you do need to change the ending い syllable to く.
Koori wa mizu o tsumetaku suru.
Ice makes water cold.
Kesa, watashi no kuruma o kirei ni shita.
I cleaned my car this morning. (Literal translation: I made my car clean this morning.)
Note: This sentence pattern can be used only for changes that were brought about by a person or thing. Changes that happen on their own are not explained using this expression. For example,「木の葉の色にした。」does not mean "The colour of the leaves changed."