Near-Synonym Field Guide: 直す, 治す, 修正する, 改善する

Core examples: 時計を直す, 病気を治す, 誤字を修正する, 発言を訂正する, 業務を改善する, 製品を改良する, 悪い癖を直す.

“Fix” is not one Japanese verb

English “fix” covers many situations:

• fix a clock,
• fix a typo,
• fix an illness,
• fix a bad habit,
• fix a process,
• fix a product design,
• fix a statement.

Japanese separates these more clearly:

直す 治す 修正する 訂正する 改善する 改良する

The key principle is:

Choose repair vocabulary by the type of problem: object, body, text, statement, process, product, or behavior.

If you use the wrong verb, the sentence may still be understandable, but the category of the problem changes.

直す: repair, correct, restore

直す is broad. It can mean to fix, repair, correct, restore, or put back into proper condition.

Examples:

時計を直す fix a clock

間違いを直す correct a mistake

悪い癖を直す fix/break a bad habit

姿勢を直す correct posture

直す is often used for everyday repair or correction. It can apply to objects, behavior, writing, posture, and habits.

Related intransitive:

直る be fixed / return to normal

治す: cure or heal

治す is for illness, injury, medical condition, or healing.

Examples:

病気を治す cure an illness

けがを治す heal an injury

風邪を治す recover from/cure a cold

Related intransitive:

治る heal / be cured

Do not use 直す for medical healing unless the context is casual or metaphorical. 病気を直す is not the standard written choice. Use 治す.

修正する: revise, modify, correct text/data/design

修正する is formal and often used for written material, data, plans, code, design, documents, or errors.

Examples:

誤字を修正する correct a typo

データを修正する correct/modify data

計画を修正する revise a plan

コードを修正する fix/modify code

It sounds more formal and process-oriented than 直す. In workplace or technical contexts, 修正する is extremely useful.

訂正する: correct a statement or published error

訂正する often means to correct something stated, announced, written, or publicly given.

Examples:

発言を訂正する correct one’s statement

記事の誤りを訂正する correct an error in an article

名前の表記を訂正する correct name spelling

訂正 carries a sense of correcting an error in information, statement, or published text. It can sound more official than 直す.

改善する: improve a process, condition, or situation

改善する means improve, especially a system, condition, process, habit, environment, or performance.

Examples:

業務を改善する improve operations

生活習慣を改善する improve lifestyle habits

状況を改善する improve the situation

品質を改善する improve quality

改善 is not just fixing a broken item. It suggests making something better.

改良する: improve a product or design

改良する often means improve through modification, especially products, tools, technology, methods, or designs.

Examples:

製品を改良する improve a product

品種を改良する improve/breed a variety

機械を改良する improve a machine

Compared with 改善, 改良 often feels more technical or design/product-oriented.

Example bank walkthrough

時計を直す

Repair a clock.

Learner action: 直す for physical repair.

病気を治す

Cure an illness.

Learner action: 治す for medical healing.

誤字を修正する

Correct a typo.

Learner action: 修正する for text/data/document correction.

発言を訂正する

Correct a statement.

Learner action: 訂正する for spoken/written information correction.

業務を改善する

Improve operations.

Learner action: 改善する for processes and conditions.

製品を改良する

Improve a product.

Learner action: 改良する for product/technical improvement.

悪い癖を直す

Fix a bad habit.

Learner action: 直す works for behavior correction.

Problem-type triage

Ask:

1. Physical object broken? 直す.
2. Body/illness/injury? 治す.
3. Text/data/code/design error? 修正する.
4. Wrong statement/public information? 訂正する.
5. Process/condition/situation? 改善する.
6. Product/tool/method design? 改良する.
7. Everyday casual correction? 直す may be natural.

Repair vocabulary by failure type

A useful way to choose among these words is to classify the failure.

This table prevents a common learner habit: using 直す for every repair-like situation. 直す is broad and useful, but formal domains often prefer narrower verbs.

修正 versus 訂正

修正 and 訂正 both involve correction, but their center of gravity differs.

修正する

Often means revise, modify, or correct something that can be edited: document, design, plan, data, code, draft.

訂正する

Often means correct an error in something stated, written, announced, or recorded.

Compare:

資料のグラフを修正しました。 I revised/corrected the graph in the materials.

先ほどの発言を訂正します。 I correct my earlier statement.

訂正 has a stronger “the previous information was wrong” feel.

改善 versus 改良

改善 often targets processes, conditions, systems, or performance.

労働環境を改善する。 improve the work environment.

改良 often targets a product, tool, method, breed, or design.

新モデルではエンジンを改良した。 In the new model, the engine was improved.

Both mean improvement, but 改善 sounds problem-condition-oriented, while 改良 sounds design/technical-development-oriented.

A strong tool for this article would classify the target of “fix.”

Suggested functions:

1. Target selector: object, illness, typo, statement, process, product, habit.
2. Verb recommendation: 直す, 治す, 修正する, 訂正する, 改善する, 改良する.
3. Register labels: casual, medical, technical, official, business.
4. Intransitive pairs: 直る, 治る, 改善される.
5. Example rewrites: casual vs formal.
6. Collocation drill: choose natural object-verb pairs.

Final rule

Do not translate every “fix” as 直す.

直す repairs and corrects broadly. 治す heals. 修正する revises text/data/plans. 訂正する corrects statements or information. 改善する improves conditions or processes. 改良する improves products or methods.

The problem type chooses the verb.