Questions & Answers about Moj brat će nam doneti tortu.
Because moj has to agree with brat in gender, number, and case.
Here, brat is:
- masculine
- singular
- nominative (because it is the subject)
So the correct form is moj brat.
If the noun changed, moj would change too. For example:
- moja sestra = my sister
- mog brata = of my brother / my brother (in some other cases)
- mom bratu = to my brother
Brat is in the nominative case.
It is the subject of the sentence, the person doing the action:
- Moj brat = the one who will bring the cake
In Serbian, the subject is usually in the nominative.
Će is the future-tense auxiliary.
More specifically, it is the 3rd person singular clitic form used to make the future:
- ja ću
- ti ćeš
- on/ona/ono će
- mi ćemo