Quick answer
mi · tu · su · nuestro
Possessives agree with the thing owned, not with the owner. And su covers his, her, your and their all at once.
The forms
| Person | Singular | Plural |
|---|---|---|
| yo | mi | mis |
| tú | tu | tus |
| él / ella / usted | su | sus |
| nosotros | nuestro / nuestra | nuestros / nuestras |
| vosotros | vuestro / vuestra | vuestros / vuestras |
| ellos / ustedes | su | sus |
Only nuestro and vuestro change for gender. The rest change for number only.
Agreement with the thing, not the owner
This is the opposite of English "his book / her book".
When su is ambiguous
If the context does not make it clear, Spanish rebuilds the phrase with de.
There is no apostrophe-s in Spanish. Ownership always runs backwards through de.
Memory trick
Count the objects, not the owners. Two books and one owner still gives you sus libros.
Typical mistakes
Nuestro casa es pequeña.
Nuestra casa es pequeña.
Casa is feminine, so nuestro must become nuestra. Only this form and vuestro carry gender.
Es el libro de Ana’s.
Es el libro de Ana.
Spanish has no possessive apostrophe at all. De plus the name is the whole construction.
Mis hermano vive aquí.
Mi hermano vive aquí.
One brother, so the possessive stays singular.
Check yourself
___ padres viven en Lima. (nosotros)