Or: La maestra va al parque con sus alumnos. Here the maestra is singular but the alumnos are plural, so it's "sus" The other responders gave pretty encyclopedic answers, but I think you wanted to zero in on "su" and "sus" Su and sus have multiple meanings: His, Hers, Your (used with Ud. or Uds.) also "Their" either gender.
What is the difference between sus and tus? Compare and contrast the definitions and English translations of sus and tus on SpanishDictionary.com, the world's most accurate Spanish-English reference website.
Please help me understand this in a very basic beginner way. I see there is a question posted already about su and sus, but I don't understand if I'm reading Spanish and the sentence contains, "su", or "sus", how can I tell if it's talking about a ma...