"Where is the school?"
Translation:Ubi est ludus?
Ludus is the singular nominative. The school is [location]. Where is the school?
Ludi can be the plural nominative (Ubi sunt ludi?, Where are the schools?), the singular genitive (magister ludi, the school's teacher), or the plural vocative ("Hey, schools! Yeah, I'm talking to multiple buildings.")
Adjectives must agree in gender, number, and case with the nouns they modify, but they have their own declensions. Sometimes you get lucky and the adjective just happens to follow the same declension as the noun, but that is not a guarantee.
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