Mi
How do you know what "su" means in a sentence since it has so many translations?
By context clues
Why do we need four different ways to say "our"?
So it can match the noun: masc/fem/sing/plural
Why aren't you able to precisely determine what this means in English: Su abuela
This is because the reader does not have a way of knowing which English definition of su is the one intended
Translate to Spanish: Our little sister
Nuestra hermanita
Translate to Spanish: Her mother is in Spain.
Su madre está en España.