Mastering [Adjective Stem] + に/く + する
for JLPT

[Adjective Stem] + に/く + する

If you are studying for the JLPT, whether you are tackling N3, N2, or even aiming for the advanced heights of N1, you know that achieving fluency goes far beyond memorizing isolated vocabulary and simple verb conjugations. You need to master the subtle art of transformation—the ability to express not just what things are, but how you change them.

Before diving into the grammar detail, here is the JLPT N4 complete guide: 
The Ultimate JLPT N4 Complete Guide: Your Gateway to Working and Living in Japan

Many students hit a frustrating wall when they try to move beyond basic descriptive language. You can say, 部屋はきれいです (The room is clean). But how do you express the active process of making the room clean? How do you say, “I will simplify the plan,” or “Please make the volume louder”?

This is where the incredibly versatile and fundamentally important grammatical structure, the act of turning an adjective or an adjectival concept into an active, transitive verb using する (suru), comes into play. This grammar point is essential for expressing intention, control, and transformation in Japanese. It allows you to shift from merely observing the world to actively shaping it.

Let’s dive deep into the mechanics of this transformation, ensuring you can use this structure flawlessly on the JLPT and in everyday communication.

Decoding the Core Grammar: When Adjectives Become Transitive Verbs

The structure we are examining involves taking a descriptive word (an adjective or adjectival noun) and combining it with the verb する (to do) to create a new verb meaning “to make [something] become [that state].”

This is a powerful conversion tool, instantly turning a state of being (e.g., clean, loud, easy) into an action (e.g., to clean, to make louder, to simplify).

The Fundamental Rule: Connecting the Adjective and する

The key to this transformation lies in the correct use of the connective particle. The particle acts as a bridge, preparing the adjective to modify the verb する.

1. Transforming Adjectival Nouns (Na-Adjectives): The 「〜にする」 Pattern

Adjectival Nouns (often called Na-Adjectives, like きれい (kirei – clean/beautiful) or 簡単 (kantan – easy)) are the most common candidates for this structure. They use the particle に (ni) to connect to する.

• Structure: [Na-Adjective Stem] + に + する

The particle 「に」 here functions as a marker for the result or the state that the object is being changed into. It essentially means “into the state of X.”

Examples:

• きれい (Kirei: clean) → きれいに する (Kirei ni suru: to make clean, to clean up)
• 簡単 (Kantan: easy) → 簡単に する (Kantan ni suru: to make easy, to simplify)
• 静か (Shizuka: quiet) → 静かに する (Shizuka ni suru: to make quiet, to quiet down)

This resulting verb phrase is always transitive (他動詞, tadoushi). This means it requires a direct object marked by the particle を (o).

Full Structure: [Object] + を + [Na-Adjective Stem] + に + する

Example: 私は 計画を 簡単に します。
(Watashi wa keikaku o kantan ni shimasu.)
I will make the plan simple (I will simplify the plan).

2. Transforming True Adjectives (I-Adjectives): The 「〜くする」 Pattern

Although the prompt focuses broadly on “Adjective + する,” you must also be aware of how true adjectives (I-Adjectives, like 高い (takai – high/expensive) or 大きい (ookii – large)) follow a similar, yet distinct, rule.

• Structure: [I-Adjective Stem] + く + する

I-Adjectives change their ending to く (ku)—which is their adverbial form—to modify the verb する.

Examples:

• 大きい (Ookii: large) → 大きく する (Ookiku suru: to make larger, to enlarge)
• 高い (Takai: high/expensive) → 高く する (Takaku suru: to make high, to raise the price)
• 速い (Hayai: fast) → 速く する (Hayaku suru: to make faster, to speed up)

Full Structure: [Object] + を + [I-Adjective Stem] + く + する

Example: 彼は 音量を 大きく しました。
(Kare wa onryou o ookiku shimashita.)
He made the volume louder (He turned up the volume).

For JLPT purposes, mastering both the 〜にする and 〜くする conversions is crucial, as they both represent the core concept of intentional transformation using する.

Real-Life Applications: Transformative Sentences You Can Use Today

Let’s look at three practical examples, focusing on the highly prevalent Na-adjective conversions (〜にする), to illustrate how this grammar structure adds precision and intention to your Japanese.

Example 1: Efficiency and Simplification (Using 簡単にする)

This structure is indispensable in professional or academic settings where improvement and efficiency are key. We often need to express the action of streamlining a process.

日本語: このレポートは複雑すぎるので、もっと簡単にしてください。

Romaji: Kono repōto wa fukuzatsu sugiru node, motto kantan ni shite kudasai.

English Translation: This report is too complicated, so please make it simpler (simplify it).

Analysis: Here, 複雑 (fukuzatsu – complicated) is the state, and 簡単にしてください is the active command to change that state. The speaker is requesting a transformation from complex to simple.

Nuance in Context: The Power of に する over a Simple Verb

Why use 簡単にしてください instead of perhaps a verb like 減らす (herasu – to reduce)?

• 簡単にしてください focuses on the resulting state: simplicity. It’s an instruction about the quality of the report.
• If you used 削減してください (sakugen shite kudasai – please reduce it), you would be focusing on the quantity, such as the length or number of pages.

The 〜にする structure gives you direct control over the qualitative aspect of the object.

Example 2: Managing Environment and Maintenance (Using きれいに/清潔にする)

Cleaning, tidying, and maintaining one’s environment relies heavily on this transitive transformation. The state of “clean” (きれい or 清潔 – seiketsu) is achieved through intentional action.

日本語: 明日、お客さんが来るから、必ず部屋をきれいにしてください。

Romaji: Ashita, okyaku-san ga kuru kara, kanarazu heya o kirei ni shite kudasai.

English Translation: Guests are coming tomorrow, so you must make the room clean (clean the room).

Analysis: 部屋を (heya o) marks the direct object (the room). きれいに (kirei ni) defines the target state (clean). してください (shite kudasai) is the command to perform the action and achieve the target state.

Expansion into Related Concepts

This principle extends to many other state-based transformations:

• 安全にする (anzen ni suru): To make safe, to ensure safety. (e.g., システムを安全にする – shisutemu o anzen ni suru)
• 自由にする (jiyū ni suru): To make free, to set free, or to give freedom. (e.g., 好きなように自由にする – suki na yō ni jiyū ni suru: to let someone do as they please)

Example 3: Decision Making and Preference (Using 特定にする)

In decision-making or narrowing down choices, the 〜にする structure is used to express actively choosing or fixing a state.

日本語: 多くの候補があったが、最終的にこのデザインに特定しました。

Romaji: Ooku no kōho ga atta ga, saishūteki ni kono dezain ni tokutei shimashita.

English Translation: There were many candidates, but finally, we specified/fixed it to this design (we chose this design).

Analysis: 特定 (tokutei – specific) is the Na-adjective stem. The decision-making process is the act of transforming the general state (many candidates) into a specific state (this design). Although the literal translation uses “specified,” in context, this is the dynamic verb of making a final, specific choice.

Note on Choice: When choosing a restaurant or item, Japanese often uses 「これにする」 (kore ni suru – I will make it this one/I will choose this one). This shows that the concept of transforming a decision into a fixed state is fundamental to this grammar point.

Navigating the Pitfalls: Common Mistakes and Nuance Clarification

While the concept of [Adjective] + する seems straightforward, there are two major areas where JLPT learners frequently stumble: confusing transitivity and mixing up the core conversion particles.

Pitfall 1: Confusing する and なる (Intentional vs. Spontaneous Change)

This is arguably the most critical distinction in Japanese grammar when dealing with change of state.

A. 〜にする (Ni Suru) / 〜くする (Ku Suru)

These forms express intentional, volitional, transitive action. You are the agent causing the change.

• Particle used for the object: を (o)
• Meaning: I make X become Y. (I clean the room.)

Example: 私は 部屋を きれいに します。
(I intentionally clean the room.)

B. 〜になる (Ni Naru) / 〜くなる (Ku Naru)

These forms express spontaneous, non-volitional, automatic change. The object changes on its own, or due to external, non-controlled factors.

• Particle used for the subject: が (ga)
• Meaning: X becomes Y. (The room becomes clean.)

Example: 部屋が きれいに なります。
(The room becomes clean—e.g., after the cleaning service leaves, or over time.)

Crucial Comparison Table:

Structure	Type of Change	Transitivity	Object/Subject Particle
〜にする / 〜くする	Intentional (Controlled by the agent)	Transitive (他動詞)	を (Target/Object)
〜になる / 〜くなる	Spontaneous (Natural, automatic)	Intransitive (自動詞)	が (Subject)

When you use 「〜にする」, you are taking responsibility for the transformation. This is essential for expressing commands, requests, plans, and active endeavors.

Pitfall 2: Mixing Up Adjective Types (に vs. く)

Another common mistake among intermediate learners is forgetting whether the adjective they are converting is an I-Adjective or a Na-Adjective, leading to particle errors.

• • Incorrect: 部屋を 大きいに する (Ookii is an I-Adjective, cannot take に)
  • Correct: 部屋を 大きく する

• Incorrect: 計画を 簡単く する (Kantan is a Na-Adjective, cannot take く)
• Correct: 計画を 簡単に する

If you are ever unsure, try putting the adjective into its adverbial form first (the form used before a verb like 走る – hashiru or 考える – kangaeru). This mental test will instantly reveal whether you need く or に:

• きれいに 走る (kirei ni hashiru – running beautifully) → Use に
• 大きく 考える (ookiku kangaeru – thinking largely) → Use く

Pitfall 3: Redundancy When a Simple Verb Exists

While 〜にする / 〜くする is grammatically correct for almost any transformation, native speakers often prefer simpler, dedicated verbs when they exist.

For example, while you could technically say:

• 電気を 明るくする (Denki o akaruku suru – to make the light bright)

A native speaker might simply say:

• 電気を つける (Denki o tsukeru – to turn on the light)

The 〜にする / 〜くする structure is most effective and natural when:

1. The transformation verb is conceptually unique (e.g., 複雑にする – to complicate).
2. You are emphasizing the resulting state over the action itself (e.g., 快適にする – to make comfortable).
3. There is no simple, single-word verb equivalent.

Elevate Your Japanese: Practical Study Tips and Conclusion

The ability to use the [Adjective Stem] + に/く + する structure effectively is a cornerstone of mid-to-advanced Japanese proficiency. It is the language mechanism that allows you to shift from merely describing static situations to actively managing and changing them.

For your JLPT preparation, here are actionable strategies:

1. Create Transformation Flashcards

Don’t just memorize the base adjective (e.g., 便利 – benri). Memorize its full transitive transformation:

• Card Front: 便利 (Benri)
• Card Back: 便利にする (Benri ni suru) → 他動詞 (Transitive) → Meaning: To make convenient/To utilize.

2. Practice the Suru vs. Naru Switch

Take any adjective you learn and consciously write two sentences immediately: one using 〜にする/くする (intentional change) and one using 〜になる/くなる (spontaneous change).

• Example: 広く (Hiroku – wide)
• する: 庭を広くする (Niwa o hiroku suru: I will enlarge the garden.)
• なる: 庭が広くなる (Niwa ga hiroku naru: The garden will become large.)

3. Pay Attention to Adjective Types

When encountering a new adjective in your studies, immediately categorize it as an I-Adjective or a Na-Adjective. This simple step will prevent the crucial error of mixing up the に and く connectors.

By integrating this dynamic transformation into your vocabulary, you are giving yourself the tools to express complex ideas, exert influence, and articulate changes that you initiate. This level of expressive capability is exactly what the JLPT examiners look for in candidates advancing to the higher levels. Master 〜にする / 〜くする, and watch your Japanese fluency transform.

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