Silvās clārās īnsulae magnae spectātis.

Question

Uses of silva clārās īnsulae and magnae

Answer 1

-as is the ending for accusative plural, but I don't ever remember seeing long marks over those a's.

http://www.thelatinlibrary.com/101/Declension123.pdf

So ask yourself, what do nouns in the accusative case (and the adjectives that modify them) do in a sentence?

-ae is the ending for genitive singular nouns (and the adjectives that modify them). What is the genitive case used for?