I'm reading Familia Romana book and encountered these 2 sentences.
Corsica et Sardinia insulae magnae sunt.
Brundisium et Sparta oppida magna sunt.
Why does the ae changes to a and vice versa?
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In Latin, nouns belong to different groups, which are called declensions. The word insula is of the first declension, whereas the word oppidum is of the second declension. Each declension has its own endings. In addition, oppidum is neuter.
Neuter words of the second declension have a singular ending -um, plural -a; words of the first declension have a singular ending on -a, and a plural on -ae. That's why it is one oppidum, two oppida; but one insula, two insulae.
By the way, the fact that both oppida (plural) and insula (singular) end on -a is best considered coincidental in the present context.