"Your sandwich is not there."
Translation:Tu sándwich no está allí.
Right. Su = his, her, its, and your (formal). (Example: Su camisa = his/her/its/your shirt.)
Tu = your (familiar). (Example: Tu camisa = your shirt.)
Familiar (tu) is what you would use with someone your age or younger than you, your friends, siblings, etc. - someone you are familiar with.
Formal (su) is a more respectful "your", so you would use it with someone you've just met, your elders, your boss, etc.
This is the same with tú (you, familiar) and usted (you, formal).
Hope this helps!