1. Los adjetivos
An adjective must agree with the word it describes (modifies) in 2 ways:
Gender (masculine/feminine)
Number (singular/plural)

2. Agreement of adjectives:
Mi amigo es alto. =
La chica es bonita. =
My friend is tall.
(Friend is masc./sing. so the adjective describing friend had to masc./sing. too.)
The girl is pretty.
(Girl is fem./sing. so the adjective describing girl had to be fem./sing. too)

3. Agreement of adjectives:
Because there is no word for “it” as a subject every noun is also considered masc. or fem.
La escuela es vieja y fea. =
The school is old and ugly.
Es un lápiz rojo. =
It is a red pencil.

4. Agreement of adjectives:
It is easy for most students to remember that an adjective has to agree in gender (masc. or fem.), but many have trouble remembering to make the adjective agree in number (sing. or pl.) as well.
La chica es bonita. = The girl is pretty. but
Las chicas son bonitas. = The girls are pretty.
El muchacho es guapo. = The boy is handsome.
Los muchachos son guapos. = The boys are handsome.
Son libros fantásticos.= They are fantastic books.

5. Agreement of adjectives:
Singular:
He is Puerto Rican.
She is Venezuelan.
The girl is very nice.
The student(fem) is blonde..
You are rather short.
The hard-working boy
A redheaded friend(fem).
Él es puertorriqueño.
Ella es venezolana.
La chica es muy simpática.
La alumna es rubia.
Tú eres bastante bajo/a.
El muchacho trabajador
Una amiga pelirroja

6. Agreement of adjectives:
Plural:
They are very ugly.
The girls are brunette.
We are not lazy.
You(for/pl) are American.
Some mean friends
The fat boys
Ellos son muy feos.
Las chicas son morenas.
Nosotros no somos perezosos.
Uds. son americanos.
Unos amigos antipáticos
Los chicos gordos