に基づいて・に沿って・に合わせて: Based On, Along, Matching | Japanese Grammar

English says "according to," "based on," "in line with," and "to match" almost interchangeably, and none of them map cleanly onto a single Japanese word. Japanese instead sorts the idea by what relationship you have to the standard. Are you grounding a decision on a foundation of evidence? That's に基づいて — 事実に基づいて, "based on the facts." Are you following a line, a route, or a policy? That's に沿って — 方針に沿って, "in line with the policy." Are you adjusting yourself to fit a moving target? That's に合わせて — 音楽に合わせて踊る, "dance to the music."

Ground it, follow it, or match it — that three-way sort is the whole page. Get it right and you will stop reaching for によって (which is agent/means, a different job entirely) every time you want to say "based on." These are formal, written-register particles; keeping them straight is a hallmark of polished written and formal Japanese.

に基づいて — grounded on an evidential basis

に基づいて (modifier form に基づく／に基づいた) cites the foundation on which something rests: facts, data, laws, evidence, a true story. The verb 基づく means "to be grounded in, to have as its basis." Ask "on what foundation?" and the answer takes に基づいて.

データに基づいて、冷静に判断するべきだ。

dēta ni motozuite, reisei ni handan suru beki da

We should judge calmly, based on the data.

この映画は実話に基づいている。

kono eiga wa jitsuwa ni motozuiteiru

This movie is based on a true story.

記者には、事実に基づいて報道する責任がある。

kisha ni wa, jijitsu ni motozuite hōdō suru sekinin ga aru

Journalists have a responsibility to report based on the facts.

容疑者は法律に基づいて処罰された。

yōgisha wa hōritsu ni motozuite shobatsu sareta

The suspect was punished in accordance with the law.

The modifier form is everywhere in formal writing, turning a basis into an adjective on the following noun:

これは憲法に基づく国民の権利だ。

kore wa kenpō ni motozuku kokumin no kenri da

This is a right of the people grounded in the constitution.

に沿って — following a line, route, or policy

に沿って (from 沿う, "to run along") means moving along something — literally along a physical line, or figuratively staying on the track of a plan, policy, or set of wishes. Ask "along what line?" The English is "along" or "in line with."

Physically:

川に沿って、ゆっくり歩いた。

kawa ni sotte, yukkuri aruita

I walked slowly along the river.

道に沿って、桜の木が植えてある。

michi ni sotte, sakura no ki ga uete aru

Cherry trees are planted along the road.

Figuratively — following a plan, policy, or request:

計画に沿って、プロジェクトを進めている。

keikaku ni sotte, purojekuto o susumeteiru

We're advancing the project along (in line with) the plan.

社員は会社の方針に沿って行動しなければならない。

shain wa kaisha no hōshin ni sotte kōdō shinakereba naranai

Employees must act in line with company policy.

ご要望に沿って、対応させていただきます。

goyōbō ni sotte, taiō sasete itadakimasu

We will respond in accordance with your request. (humble/business)

The image is a line you stay on: a river, a road, a plan, the shape of someone's wishes. You are not grounding on it (that's 基づいて) — you are tracing along it.

に合わせて — adjusting to fit a target

に合わせて (from 合わせる, "to bring into agreement, to fit together") means you change yourself to match something — the rhythm, the other person, the budget, the time. The target sets the shape; you conform to it. Ask "to fit/match what?"

みんなで音楽に合わせて踊った。

minna de ongaku ni awasete odotta

Everyone danced to (in time with) the music.

子供たちがリズムに合わせて手をたたいている。

kodomotachi ga rizumu ni awasete te o tataiteiru

The children are clapping in time with the rhythm.

先生は生徒のレベルに合わせて話し方を変える。

sensei wa seito no reberu ni awasete hanashikata o kaeru

The teacher changes how she speaks to match the students' level.

限られた予算に合わせて、計画を立て直した。

kagirareta yosan ni awasete, keikaku o tatenaoshita

We reworked the plan to fit the limited budget.

The literal use — physically setting something to a target — makes the "adjust to match" core vivid:

時計を正しい時間に合わせた。

tokei o tadashii jikan ni awaseta

I set the clock to the correct time.

Note the human-relations idiom 相手に合わせる, "to accommodate / go along with the other person" — bending your own preferences to fit theirs. It is a deeply cultural verb.

いつも自分の意見を言わず、相手に合わせてばかりいる。

itsumo jibun no iken o iwazu, aite ni awasete bakari iru

He never states his own opinion and is always just accommodating others.

The three-way sort

Particle	Core relationship	Ask yourself	Typical partners
に基づいて	grounded on an evidential basis	"on what foundation?"	事実, データ, 法律, 実話
に沿って	following a line / route / policy	"along what line?"	川, 道, 計画, 方針, 要望
に合わせて	adjusting to fit a target	"to match what?"	音楽, リズム, 相手, 予算, 時間

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One quick test separates 沿って from 合わせて: with 沿って you move along a fixed line (you don't change the line); with 合わせて you reshape yourself to fit a target. You walk 川に沿って (the river doesn't bend for you) but dance 音楽に合わせて (you bend to the music).

に基づいて vs によって — grounding vs agent

The brief's headline trap: English "based on" tempts learners into によって, but によって marks an agent or means ("by / through"), not a foundation. The difference is not subtle once you see what によって does to the sentence — it can make the "basis" look like the author:

❌ この小説は実話によって書かれた。

kono shōsetsu wa jitsuwa ni yotte kakareta

Wrong — this reads 'written BY the true story,' as if the true story were the author.

✅ この小説は実話に基づいて書かれた。

kono shōsetsu wa jitsuwa ni motozuite kakareta

This novel was written based on a true story.

For "grounded in / based on," always use に基づいて; save によって for the agent, means, or cause.

The casual equivalent: をもとに

に基づいて is formal. In everyday speech, the lighter 〜をもとに(して) covers much of the same "based on / using X as a base" ground:

この話をもとに、脚本を書いた。

kono hanashi o moto ni, kyakuhon o kaita

I wrote the screenplay based on this story.

をもとに emphasizes X as raw material you built from; に基づいて emphasizes X as the justifying foundation you can't stray from. For laws and evidence, prefer に基づいて.

Common Mistakes

1. Using によって for "based on." It marks agent/means, not a foundation — see the 実話 example above. Grounding takes に基づいて.

❌ 事実によって記事を書くべきだ。

jijitsu ni yotte kiji o kaku beki da

Off — this suggests the facts are the means/agent; for 'based on the facts' use に基づいて.

✅ 事実に基づいて記事を書くべきだ。

jijitsu ni motozuite kiji o kaku beki da

One should write articles based on the facts.

2. Mixing に沿って and に合わせて. You follow along a fixed line (沿って); you reshape to match a target (合わせて). Don't swap them.

❌ 音楽に沿って踊る。

ongaku ni sotte odoru

Wrong — you don't trace 'along a line of' music; you match its rhythm, so 合わせて.

✅ 音楽に合わせて踊る。

ongaku ni awasete odoru

Dance in time with the music.

3. Using に合わせて where you mean "grounded on." 合わせて implies bending to fit — which, with evidence, wrongly suggests you're tailoring the judgment to the data rather than deriving it from it.

❌ データに合わせて判断する。

dēta ni awasete handan suru

Off — implies fudging the judgment to fit the data; for an evidence-based decision use に基づいて.

✅ データに基づいて判断する。

dēta ni motozuite handan suru

Judge based on the data.

4. Using に沿って for a river as if you were matching it. The physical "along" is に沿って, not 合わせて.

❌ 川に合わせて歩いた。

kawa ni awasete aruita

Wrong — walking beside a river is 川に沿って; 合わせて would mean adjusting yourself to the river.

✅ 川に沿って歩いた。

kawa ni sotte aruita

I walked along the river.

Key Takeaways

Three "according to"s, sorted by your relationship to the standard: ground it, follow it, or match it.
に基づいて (に基づく) = grounded on an evidential basis — facts, data, laws, a true story.
に沿って = following a line, route, or policy — physical (川に沿って) or figurative (方針に沿って).
に合わせて = adjusting yourself to fit a target — 音楽に合わせて踊る, 相手に合わせる.
Never use によって for "based on" (it marks agent/means); in casual speech, 〜をもとに is the lighter stand-in for に基づいて.

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