Gender and Agreement
In English, nouns have gender. Adjectives do not change when we modify nouns of different genders.
e.g. pretty girl, pretty boy, pretty dog, pretty paintings
Although the noun changed gender, the adjective did not. In every case, the adjective remained the same regardless of the gender of the noun.
In Latin (and all Romance languages), adjectives have to agree with the nouns they modify in gender.
e.g. pluma bona (the good pen), stilus bonus (the good pencil), puer bonus (the good boy)
In these examples, the adjectives changed gender along with the noun. There is a big exception -
Nouns can end in -a, -us, or -er.
Adjectives do not have the -er ending. Adjectives can only end in -a or -us. Therefore, a masculine noun that ends in -er will have an adjective that ends in -us.
When and adjective and a noun have the same gender, they are in agreement.
*If you think about Spanish words, you can see adjectives change gender. e.g Costa Rica and Puerto Rico. The adjective has to agree with the noun it modifies in gender.