Learn Spanish (Spain): La cuarta puerta a la derecha es el baño. - The fourth door on the right is the bathroom.

Ordinal numbers (first, second, third, etc.) act like adjectives in Spanish, which means they must match the gender of the noun they describe. Because puerta (door) is feminine, cuarto changes to cuarta.

Why does cuarta come before puerta when most Spanish adjectives come after the noun?

While descriptive adjectives (like colors or sizes) usually go after the noun, ordinal numbers like cuarta almost always go before the noun they are counting.

Why does this sentence use es instead of está if it tells us where the bathroom is?

You use estar to talk about a location (like El baño está a la derecha). However, this sentence is identifying what the door is. Because we are equating two things (The door = the bathroom), we must use ser (es).

I learned that derecho means straight ahead. Why does it say a la derecha here?

Derecho (used as an adverb) does mean straight ahead when giving directions. But la derecha (a feminine noun) means the right side. To say on the right or to the right, you always use the set phrase a la derecha.