There are 3 regular groups of adjectives, which agree with the noun they are describing in gender and number:
Group 1: -o, -a, -os, -as| masculine | feminine | |
| singular | un chico sincero - A sincere boy | una chica sincera - A sincere girl |
| plural | chicos sinceros - Sincere boys | chicas sinceras - Sincere girls |
| masculine | feminine | |
| singular | un chico responsable - A responsible boy | una chica responsable - A responsible girl |
| plural | chicos responsables - Responsible boys | chicas responsables - Responsible girls |
| masculine | feminine | |
| singular | un bolígrafo azul - A blue pen | una camiseta azul - A blue t-shirt |
| plural | bolígrafos azules - Blue pens | camisetas azules - Blue t-shirts |
| masculine | feminine | |
| singular | un chico trabajador - A hardworking boy | una chica trabajadora - A hardworking boy |
| plural | chicos trabajadores - Hardworking boys | chicas trabajadoras - Hardworking girls |
| masculine | feminine | |
| singular | un chico inglés - An English boy | una chica inglesa - An English girl |
| plural | chicos ingleses - English boys | chicas inglesas - English girls |
Irregular adjectives
Optimista
Un chico optimista - An optimistic boy
Chicos optimistas - Optimistic boys
Naranja
Libros naranja - Orange booksRosa
Un pantalón rosa - Pink trousers

Practice: Adjectives and How They Agree 1 Adjectives and How They Agree 2 Adjectives and How They Agree 3