Questions & Answers about Mój brat mieszka obok.
Because mój has to agree with brat.
Here:
- brat = masculine singular
- it is the subject of the sentence
- so it is in the nominative case
That means the possessive adjective mój also appears in its masculine singular nominative form: mój.
A few related forms are:
- moja siostra = my sister
- moje dziecko = my child
- moi bracia = my brothers
So mój brat is the correct matching form.
Brat is in the nominative case.
It is the subject of the sentence—the person who lives—so Polish uses the nominative.
The basic dictionary form is:
- brat = brother
In this sentence, that form stays unchanged because it is the subject:
- Mój brat mieszka obok.