Questions & Answers about Mater olivas in foro emit.
Because mater is the subject of the sentence (“mother” is doing the buying). Subjects are usually in the nominative case, and mater is the nominative singular form.
Olivas is accusative plural, because it is the direct object: it answers “what did she buy?” → olives.
It comes from oliva, olivae (1st declension feminine). The accusative plural ending for 1st declension is -ās, so oliva → olivās.
The Latin form tells you the number: olivas is explicitly plural (“olives”), while olivam would mean “an olive / one olive” (accusative singular). Latin often uses the plural for things typically bought in quantities.
in foro expresses location: “in the forum / at the marketplace.”
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