決して〜ない details

never + write + not

kesshite kakanai

never; by no means — a flat denial the speaker stakes a judgement or a promise on

meaning: never write

一度いちどした約束やくそくは、決してけっして破らやぶらない。

ichido shita yakusoku wa, kesshite yaburanai.

A promise once made is one I never break. neutral

このことは決けっして誰だれにも言いわないでね。

kono koto wa kesshite dare ni mo iwanaide ne.

Don't ever tell anyone about this, OK? casual

ない-form
i-adjective stem
na-adjective
noun
alternant

Examples

never; not ever — a categorical denial about all occasions, typically a promise, a rule or a warning

一度いちどした約束やくそくは、決けっして破やぶらない。

ichido shita yakusoku wa, kesshiteyaburanai.

A promise once made is one I never break. neutral

このことは決けっして誰だれにも言いわないでね。

kono koto wa kesshite dare ni mo iwanaide ne.

Don't ever tell anyone about this, OK? casual

お預あずかりした個人こじん情報じょうほうを、決けっして外部がいぶに開示かいじいたしません。

o azukari shita kojin jouhou o, kesshite gaibu ni kaiji itashimasen.

We will never disclose the personal information entrusted to us to any outside party. formal

by no means; not at all — heading off a judgement the listener looks about to make

この値段ねだんは決けっして安やすくない。

kono nedan wa kesshite yasukunai.

This price is by no means cheap. neutral

うちの会社かいしゃ、決けっして大おおきくないけど、居心地いごこちはいいよ。

uchi no kaisha, kesshite ookikunai kedo, igokochi wa ii yo.

Our company isn't big by any stretch, but it's a comfortable place to be. casual

今回こんかいの試験しけんは決けっして簡単かんたんではありませんでした。

konkai no shiken wa kesshite kantan de wa arimasen deshita.

This time's exam was by no means easy. neutral

今いまの話はなしは決けっして冗談じょうだんではない。

ima no hanashi wa kesshite joudan de wa nai.

What I have just said is by no means a joke. neutral

Formation

決して{V-nai}attaches to ない-form
決して{I-stem}くないattaches to i-adjective stem
決して{NA-stem}ではないattaches to na-adjective
決して{N}ではないattaches to noun

Variant notes

けっして — the kana spelling, common in casual writing and in dialogue; the kanji 決 is the norm in print.

Notes

決して commits the speaker to the denial, so it reads as a pledge or a firm
verdict rather than a measurement. Unlike 全然 it never takes a positive
complement — ×決けっして安やすい is simply not Japanese.

Related patterns

Similar

全然〜ない
全然 is the spoken absolute and has drifted into positive reassurance (全然ぜんぜん大丈夫だいじょうぶ); 決して stays negative-only and adds the speaker's commitment — 決して忘わすれない is a pledge about all future time, where 全然忘わすれない merely reports a fact.
少しも〜ない
少しも denies the smallest DEGREE of a state — 少すこしも痛いたくない leaves not even a twinge — while 決して denies the OCCASION across all future time, which is why it sits so naturally in promises, warnings and rules.
全く〜ない
Both are emphatic and written-leaning, but 全まったく measures the quantity of a state ("none of it"), while 決して measures the speaker's resolve about it — 全く知しらない claims ignorance, 決して知しらせない promises silence.