に決まっている|にきまっている
- 1States a confident prediction that something will certainly happen, as in 'is bound to' or 'is sure to'.Ex. 1
- 2Asserts that something is obviously or naturally true, as in 'of course' or 'obviously'.Ex. 2
Verb (plain form)The plain (non-ます) forms of a verb: affirmative or negative, non-past or past.
行く・行かない・行った・行かなかった
The dictionary form is only one of these: 行く.
i-adj. (plain form)The plain (non-です) forms of an i-adjective: affirmative or negative, non-past or past.
高い・高くない・高かった・高くなかった
The dictionary form is only one of these: 高い. The polite version adds です to the plain form.
na-adj. stemA na-adjective without な, i.e. the form listed in a dictionary.
静か・きれい・好き
Add な before a noun (静かな部屋); as a predicate, follow it with だ／です (静かだ).
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Mainly used in conversation to assert the speaker's own firm, often emotional conviction, frequently with sentence-final particles such as よ or じゃない. Compared with に違いない, which presents a conclusion drawn from reasoning, に決まっている sounds more subjective and assertive. It is typical of casual speech and less suited to formal writing.
Do not insert だ after な形容詞 or nouns; say 便利に決まっている, not 便利だに決まっている.
Note that it sounds strongly assertive and subjective, so it can come across as pushy toward superiors or in formal situations.
Learners should avoid it in formal writing, where に違いない or に相違ない is more appropriate.
- 1.に違いないN3「に違いない」 focuses on confident inference based on evidence and works in both speech and writing, with the polite form 「に違いありません」. When asserting an inevitable result the two are often interchangeable, but swapping is unnatural when emphasizing 'obviously, needless to say'.
- 2.に相違ないN2「に相違ない」 also expresses a confident assertion but is written language, seen mainly in papers, reports, and speeches; it sounds stiff, lacks the 'needless to say' nuance, and 「である」 may follow nouns and な-adjectives. It is not suitable as a replacement in everyday conversation.
- 3.わけがないN3「わけがない」 takes の or である after nouns and is also used in writing; it asserts 'there is no way that...', and is close to the negative form of 「に決まっている」: 「来るわけがない」≈「来ないに決まっている」.
- 4.はずだN4「はずだ」 infers 'should be...' from evidence, is stylistically neutral, and is common even in formal settings; 「はずだった」 can also express an expectation that fell through. In terms of connection, な-adjectives and nouns require 「な」/「の」, whereas 「に決まっている」 attaches directly.
- 5.だろうN5「だろう」 expresses general conjecture with less certainty than 「に決まっている」, is stylistically neutral, and appears in essays and news; with a rising intonation it can also ask the listener for confirmation. When expressing conjecture, swapping changes the degree of certainty, so they should not be interchanged freely.
- 6.っこないN2「っこない」 exclusively expresses strong negation, 'absolutely impossible', and attaches only to the ます-stem of verbs; 「に決まっている」 can attach to verbs, adjectives, and nouns to make an assertion. 「解けないに決まっている」 and 「解けっこない」 are close in meaning.
Unlike に違いない (a conviction reached by inference), the formal, written に相違ない, and はずだ (which can also be confident but presents the conclusion as following logically from known grounds), に決まっている asserts the outcome as a foregone conclusion that admits no doubt, often emotionally and without stating reasons; it is also much stronger than だろう (plain conjecture). While わけがない and っこない (casual, attached to the 動詞連用形, e.g. 行きっこない) always declare that something is impossible or will not happen, に決まっている can confidently assert either a positive or a negative outcome (失敗するに決まっている, 来ないに決まっている).
