Structure
su + noun [singular]
sus + noun [plural]
Description
Possessives, as their name suggests, indicate possession or belonging. They always agree in gender and number with the noun they refer to; that is, with the thing that is possessed. Here "your" refers to both "you - singular".
Examples
su gato negro
your [polite] black cat
su café y su agua
your [polite] coffee and your [polite] water
Extra Reading