* Where someone or something is located. Only used with the names of cities, towns, and small islands (proper nouns such as "Rome", not common nouns such as "city") plus the three common nouns "humus (ground)", "rus (countryside/farm)", and "domus (house/home)". These do not take any preposition. All other locations take a preposition and the ablative case.
Ablative vs nominative.
Here is a plain-English overview of what the cases are and how they work:
Latin cases, in English
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.