に決(き)まっているSurely, Bound to, Certainly, Be decided

確かに and に決まっている both express a strong degree of certainty, but differ in usage and nuance. 確かに is often used to concede or acknowledge something as true, and can at times be followed by a contrast, as in 確かに高いが、品質もいい (It’s certainly expensive, but the quality is good). に決まっている conveys confident personal judgment or obviousness, as in 彼がやったに決まっている (He definitely did it).3

Both にする and に決まっている relate to decisions but differ in use. にする expresses making a choice, often in daily contexts like picking what to eat, similar to 'I'll go with X.' に決まっている, despite using 決まる, often just shows strong belief, meaning 'It's surely X' or 'It's bound to be X,' rather than an actual decision.

に違いない and に決まっている both express strong conviction, but differ in tone and basis of judgment. に違いない implies a strong conclusion drawn from evidence or context, e.g. '何かあったに違いない' ('Something must have happened'). に決まっている conveys more assertive confidence, often based on common sense or the obvious, e.g. '美味しいに決まっている' ('Of course it tastes good').

These patterns are related in that both center around the idea of something being decided. ことになる expresses that something has been decided or will happen, often beyond the speaker's control, e.g. 転勤することになった ('It's been decided that I'll be transferred'), while に決まっている expresses that something is decisively true, e.g. 勝つに決まっている ('They'll definitely win').