Possessive determiners: mi, tu, su, nuestro

What it is

Possessive determiners (posesivos átonos) say whose something is: mi casa (my house), tu hermano (your brother), nuestra familia (our family). They go before the noun, take no article, and agree with the thing owned, not with the owner.

How to form it

• mi, tu, su change only for number: mi hermano, mi hermana, mis hermanos.
• nuestro changes for gender and number, like an adjective in -o: nuestro barrio, nuestra calle, nuestros vecinos, nuestras vecinas.
• The -s depends on the thing, not on the owner. Sus hijos can be one woman's children; su casa can be the house of a whole family.
• In Spain, vuestro, vuestra, vuestros, vuestras is "your" for vosotros. In Latin America, "your" for several people is su, sus or de ustedes.

Nothing goes before the possessive: no el, la, un. Mi casa, never la mi casa.

When to use it

• Family and people: Mi mamá es de Quito y mi papá es de Lima.
• Things and places you have: ¿Tu celular es nuevo? Nuestro departamento está en el centro.
• Formal "your": with usted, "your" is su: Señora Paz, ¿cuál es su dirección?
• A named owner: Spanish has no 's, it uses de: el carro de Andrés, la casa de mis abuelos.

su can mean his, her, its, their or your (formal). Usually the context makes it clear. When it doesn't, use the article + noun + de él, de ella, de usted, de ellos, de ellas, de ustedes:

Tomás y Carla viven en Rosario. La casa es de ella y el carro es de él.

• Mi hermana vive en Guadalajara con su esposo.My sister lives in Guadalajara with her husband.su = her
• —¿Tu papá es de Bogotá? —No, es de Medellín.—Is your dad from Bogotá? —No, he's from Medellín.tú → tu
• Nuestra casa está cerca del mar.Our house is near the sea.nuestra + feminine noun
• Señora Quispe, ¿cuál es su número de teléfono?Mrs Quispe, what is your phone number?usted → su
• Lucía tiene dos hermanos. Sus hermanos viven en Montevideo.Lucía has two brothers. Her brothers live in Montevideo.sus: several brothers
• Mis abuelos tienen una finca. Su finca es pequeña.My grandparents have a farm. Their farm is small.su: one farm

Contrasts and traps

• tu or tú? Without an accent, tu is "your": tu casa. With the accent, tú is "you": ¿Tú vives aquí? Both in one sentence: ¿Tú tienes tu pasaporte?
• No article before the possessive, but the article stays with de: mi casa, la casa de Ana.
• Agree with the thing: Ana y su hermano (one brother), mis tíos y su carro (one car). Many owners do not make su plural.
• nuestro agrees in gender with the thing: nuestra escuela, nuestro gato, even if all the owners are women.
• Keep the register: if you say tú to someone, their things are tu, tus; with usted, they are su, sus.
• de él does not contract: el carro de él. Only the article el contracts: el carro del profesor.

If you also speak Portuguese

Drop the article (a minha mãe → mi mamá), and remember that Brazilian seu, sua for você becomes tu with friends: tu casa. Dele, dela become su or de él, de ella, and nosso, nossa become nuestro, nuestra.