grammar explanation

The phrase, Tror du at det blir pent vær i morgen?

I know it is talking about the weather but why can it not be "tror du at været blir pen i morgen?" is there a grammar rule i don't understand or am it mistranslating this sentence.

"tror du at været blir pent i morgen" would be an equally correct sentence. However, you have to change "pen"->"pent" since it is referring to "været" which has neutral gender!