1. Gender Agreement (Masculino vs. Femenino)

Adjectives must match the noun they describe in gender.

• Adjectives ending in -o change to -a for feminine.
  Example: el libro rojo (the red book) | la casa roja (the red house).
• Adjectives ending in -e or consonants are neutral (unisex).
  Example: el chico inteligente / la chica inteligente | el coche azul / la bicicleta azul.

2. Number Agreement (Singular vs. Plural)

Adjectives must also match the noun in number (singular or plural):

• If the adjective ends in a vowel, add -s.
  Example: inteligente → inteligentes | rojo → rojos.
• If the adjective ends in a consonant, add -es.
  Example: azul → azules | difícil → difíciles.
• If the adjective ends in -z, change -z to -c and add -es.
  Example: feliz → felices.

3. Special Case: Nationalities (Nacionalidades)

Adjectives of nationality that end in a consonant (like -és, -ol, -án) add an -a for the feminine form, and written accents are often dropped:

• inglés (masc. singular) → inglesa (fem. singular)
• español (masc. singular) → española (fem. singular)
• alemán (masc. singular) → alemana (fem. singular)

4. Accent Shifts (Cambios de acento)

Some adjectives gain or lose a written accent mark when pluralized to keep the natural stress on the correct syllable.
• joven (singular) → jóvenes (plural).