Questions & Answers about Moja sestra je odlučila da ostane kod bake i dede.
Moja sestra is in the nominative case because it is the subject of the sentence.
- sestra = sister
- moja = my
The adjective/pronoun moja has to agree with sestra in gender, number, and case, so both are feminine singular nominative.
If you said moju sestru, that would be accusative, which is used for a direct object, not the subject.
Je is an unstressed auxiliary verb, and in Serbian these little unstressed words usually go in the second position of the sentence or clause.
So the neutral order is:
Moja sestra je odlučila...
Here, Moja sestra acts as the first unit, and je comes right after it.
This is a very common Serbian pattern with clitics such as je, sam, si, se, ga, mi, etc.
Je odlučila is the normal way to form the past tense in Serbian.
It consists of:
- the auxiliary je = is/has (here part of the past-tense structure)