When numbers start agreeing

Hola Sidney S.

Great question, this is a very common point of confusion.

In Spanish, numbers do not generally agree with the noun. The noun itself takes the plural, but the number stays the same:

So it’s not about being above or below 20 — simple cardinal numbers (2, 3, 14, 500, etc.) never change form to agree with the noun.

The important exception is with numbers that behave like adjectives:

• uno / una → agrees in gender

  un libro / una casa

• numbers ending in -uno (21, 31, etc.) 

  veintiún libros / veintiuna casas

• cien / ciento

  cien libros / cien casas

• compound hundreds (doscientos, quinientas, etc.) → agree in gender

  quinientos libros / quinientas casas

So what you’re seeing with quinientos libros is not the noun “agreeing with the number”, but the hundreds adjective agreeing with the noun.

The takeaway:

• Most numbers = no agreement

• Uno, numbers ending in -uno, and hundreds = agreement

Hope that helps clarify it!

Best wishes,
Silvia