〜に決まっている
Definitely, obviouslymachine translation
FormationPlain form + に決まっている (nouns・な-adjectives attach directly, without だ)
〜に決まっている asserts that something is certain, a given, as if it were "already decided": 毎日練習すれば、上手になるに決まっている (if you practice every day, of course you'll get better). It's a colloquial, blunt, emotionally charged way of speaking — often used to brush aside opposing views: そんな話はうそに決まっているよ (that story is obviously made up).
Connection: plain form + に決まっている; nouns and な-adjectives attach directly, without だ. The polite form is に決まっています.
Compared with 〜に違いない in the same group: に違いない is a rather calm statement of conviction, leaning toward writing and inner monologue, while に決まっている is livelier and more forceful, like the Vietnamese "còn phải hỏi, đương nhiên là thế!" ("need you even ask — of course it is!"). Because of its strong tone, think twice before using it with superiors — it can sound like you're talking back.
Examples
毎日練習すれば、上手になるに決まっている。
If you practice every day, you're bound to get better.
そんな話はうそに決まっているよ。
A story like that is obviously a lie.
そんなに高い店は、学生には無理に決まっていますよ。
A restaurant that expensive is obviously beyond a student's means.