に決まっている
文法の解説は英語です。
用法
(V/いA/なA~~-だ~~/N~~-だ~~) + に決まっている
Used to express a conjecture about someone definite outcome based on the speaker's conviction
Can be used if there is no solid basis for believing this is the case
例文
紙に書いても忘れるに決まっている
He is sure to forget even if he writes it down on paper
医者になりたいけど、無理に決まっている
I want to be a doctor, but I'm definitely not cur out for it