In short

なる naru means “become” - something changes on its own; する suru means “make it so” - somebody decides. Both attach to the adverbial form of the word: い-adjectives take く (安くなる yasuku naru, “get cheaper”), while な-adjectives and nouns take に (静かになる shizuka ni naru, “go quiet”; 医者になる isha ni naru, “become a doctor”). Swap なる for する and the same phrase becomes deliberate: 安くする yasuku suru is “make it cheaper”.

The rule

Step 1 - Put the word into its adverbial form. You already built these in lesson 8: い-adjectives end in く, な-adjectives and nouns take に. That single form feeds both verbs, so there is really only one rule to learn here.

• い-adjective: drop the final い, add く -> 暑い atsui -> 暑く atsuku
• な-adjective: add に -> 静か shizuka -> 静かに shizuka ni
• Noun: add に -> 医者 isha -> 医者に isha ni

Step 2 - Add なる for a change that simply happens. The pattern is い-adj-く + なる and な-adj／Noun + に + なる. Nobody is responsible; the situation moves by itself.

暑くなりました。 (Atsuku narimashita.) - It has got hot. 兄は医者になりました。 (Ani wa isha ni narimashita.) - My older brother became a doctor.

なる is a regular 五段 verb, so the tail behaves normally: なります／なりました／なりません／なりませんでした. All the tense sits on なる - the く／に part never changes.

The one irregular base is いい ii (“good”). Like every other inflected form of いい, the adverb comes from よい: よく yoku, never ×いく. So “get better” is よくなる yoku naru.

Step 3 - Add する for a change somebody brings about. Same slot, different verb: い-adj-く + する and な-adj／Noun + に + する. Now there is an agent, and the thing being changed is marked with を.

音を大きくしてください。 (Oto o ōkiku shite kudasai.) - Please turn the sound up. 部屋をきれいにしました。 (Heya o kirei ni shimashita.) - I tidied the room up.

The なる／する opposition is the heart of the lesson, and it runs right through the language: なる describes the world, する describes your intervention. English often hides this behind one verb (“the price dropped” / “we dropped the price”), so translate the situation, not the English word.

Step 4 - Negatives: 〜なくなる, “stop being / stop doing”. The negative ending ない is itself an い-adjective, so it follows the い-adjective rule exactly: ない -> なく + なる.

• 分からない wakaranai -> 分からなくなる wakaranaku naru - “come not to understand”
• 行かない ikanai -> 行かなくなる ikanaku naru - “stop going”
• 高くない takakunai -> 高くなくなる takaku naku naru - “stop being expensive”

This is the normal Japanese way of saying “no longer” or “stopped -ing”. 学生ではなくなりました (Gakusei de wa naku narimashita) - “I am no longer a student.”

Step 5 - N + にします for choosing. With a noun, する very often means pick this one rather than physically alter anything. It is the standard verb for ordering in a restaurant and for settling on an option.

私はコーヒーにします。 (Watashi wa kōhī ni shimasu.) - I’ll have coffee.

Note the particle: に, not を. コーヒーをします would mean “do coffee”, which is nonsense. And the なる counterpart is how Japanese announces a decision impersonally: 会議は来週になりました (Kaigi wa raishū ni narimashita) - “The meeting has been moved to next week”, with no mention of who moved it. Japanese reaches for なる in announcements far more readily than English reaches for “become”.

Gradual change: 〜てくる and 〜ていく (preview). Stack the て-form of なる onto くる／いく to show a change unrolling over time: 寒くなってきました (Samuku natte kimashita) - “it has been getting colder (up to now)”, 分からなくなってきました (Wakaranaku natte kimashita) - “I am starting to lose the thread”. くる looks back to the present, いく looks forward. Full treatment in Directional aspect 〜ていく／〜てくる.

Two relatives that are not yours yet. 〜ようになる／〜ようにする attach to a verb rather than an adjective and cover acquired ability and deliberate habit; they belong to Purpose and deliberate change: ために, ように. 〜ことになる／〜ことにする (“it has been decided” / “I have decided”) are built on the formal noun こと and are handled in Formal nouns こと, もの, ところ. Recognise them; do not try to use them from this lesson.

Forms & patterns

What goes in front of なる／する.

なる versus する - the same slot, two different worlds.

The tail carries the tense. なる is 五段, する is irregular; neither the く nor the に ever moves.

Examples

Common mistakes

• 寒いになりました。 (Samui ni narimashita.) -> 寒くなりました。 (Samuku narimashita.) - An い-adjective never takes に. Drop the い and add く: 寒い -> 寒く.
• この町は静かくなりました。 (Kono machi wa shizukaku narimashita.) -> この町は静かになりました。 (Kono machi wa shizuka ni narimashita.) - な-adjectives have no く form at all. They behave like nouns, so they take に.
• 天気がいくなりました。 (Tenki ga iku narimashita.) -> 天気がよくなりました。 (Tenki ga yoku narimashita.) - いい keeps its own shape only where it is uninflected (いいです, いい天気); every inflected form is built on よ: よく, よかった, よくない.
• 日本語が分からないになりました。 (Nihongo ga wakaranai ni narimashita.) -> 日本語が分からなくなりました。 (Nihongo ga wakaranaku narimashita.) - ない is an い-adjective, so it inflects like one: ない -> なく. There is no ×ないに.
• 私はコーヒーをします。 (Watashi wa kōhī o shimasu.) -> 私はコーヒーにします。 (Watashi wa kōhī ni shimasu.) - Choosing takes に. を would mark コーヒー as something you perform, which is not a possible reading.

Tips

• One form, two verbs. Learn the adverbial form once (く for い-adjectives, に for everything else) and you get both patterns free. Then the only decision left is なる (“it happened”) versus する (“I did it”) - and that decision is about meaning, never about grammar.
• Japanese prefers なる where English prefers a person. Shops say 千円になります (Sen’en ni narimasu) for “that comes to 1000 yen”, offices say 中止になりました (Chūshi ni narimashita) for “it’s been cancelled”. Naming the decider sounds blunt; letting the situation change by itself is the polite default.
• になります is not extra politeness. Beginners hear it in shops and start bolting it onto plain statements: ×こちらがメニューになります (Kochira ga menyū ni narimasu, “this becomes the menu”) is famously criticised as fake-polite Japanese. Nothing is changing, so the sentence needs です: こちらがメニューです。
• Watch the particle pair. With する the changed thing takes を (音を大きくします oto o ōkiku shimasu, “I turn the sound up”), because する is transitive. With なる there is no を at all - the thing that changes is the subject: 音が大きくなります (oto ga ōkiku narimasu, “the sound gets louder”). Seeing a stray を is the quickest way to spot that you have picked the wrong verb.