Quick answer
una casa blanca
Adjectives come after the noun and copy its gender and number.
Position and agreement
un coche rojoa red car
una casa rojaa red house
unos coches rojosred cars
unas casas rojasred houses
Adjectives ending in -e or a consonant
They do not change for gender, only for number.
un libro interesante · una película interesantean interesting book · an interesting film
un examen difícil · unos exámenes difícilesa hard exam · hard exams
When position changes the meaning
un hombre grandea big man
un gran hombrea great man
mi viejo amigomy old friend — long-standing
mi amigo viejomy friend who is old
Memory trick
English puts the adjective first and never changes it. Spanish does the opposite on both counts.
Typical mistakes
una roja casa
una casa roja
The adjective follows the noun.
las casas blanco
las casas blancas
It has to match in gender and number.
Check yourself
Las flores son ___.