Explanation
Express certainty; 'it is obvious', 'it must be', 'no doubt'.
Formation
Verb-casual + に決まっている, い-Adjective + に決まっている, な-Adjective + だに決まっている, Noun + だに決まっている
Detailed explanation
The ～に決まっている grammar point is used to express certainty or a strong conviction about a situation or event. It can be translated in English as 'it is obvious', 'it must be', or 'no doubt'. The formation differs depending on whether it is used with a verb, い-adjective, な-adjective, or noun.
Examples
Konna ni atsui hi ni undou shitara, ase wo kaku ni kimatte iru.
It's obvious that you'll sweat if you exercise on such a hot day.
Kare ga shiken ni ochitara, gakkari suru ni kimatte iru.
If he fails the exam, it's obvious he will be disappointed.
Kanojo wa itsumo okureru kara, kyou mo okureru ni kimatte iru.
She's always late, so it's obvious she'll be late today too.
Kono geemu wa tanoshii ni kimatte iru.
It's obvious that this game is fun.
Practice
Grammar content sourced from hanabira.org.