〜に決まっている（にきまっている）

About

～に決きまっている is used to say that a given piece of information is "decidedly X", and to make suppositional statements when the speaker is very confident in their belief/guess/conjecture about a given topic.

We can approximate ～に決きまっている in English as "X is surely.../has got to be.../is decidedly..." or "of course...".

～に決きまっている can attach directly to casual form verbs, い-adjectives/な-adjectives, and nouns.

E.g. 毎日まいにちシナモンロールばかり食たべていたら、太ふとるに決きまっているでしょ？ "Isn't it obvious that you'll gain weight if you eat cinnamon rolls every day?"

E.g. これを見みたら、妹いもうとも絶対ぜったい同おなじものを欲ほしがるに決きまっている。 "If my younger sister sees this, it's guaranteed that she's going to want the same one."

E.g. 10年ねん連続れんぞくで優勝ゆうしょうしているんだ。強つよいに決きまってるよ。 "(They've) won 10 years in a row. They're guaranteed to be strong."

～に決きまっている is mainly used in spoken casual contexts. ～に決きまっています is possible in polite scenarios.

Summary

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  ～に決きまっている is used to say that a given piece of information is "decidedly X"

• 

  ～に決きまっている can attach directly to casual form verbs, い-adjectives/な-adjectives, and nouns

• 

  ～に決きまっている is mainly used in spoken casual contexts