mi=my mío=mine tu=your tuyo=yours su=his/her We use the plurals (mis, míos, sus, tuyos, sus), when the object you are talking about is plural. For example: Do you know my father?-->¿Conoces a mi padre? Do you know my parents?-->¿Conoces a miS padreS? "Se", is a different word, you can use in many different ways (impersonal sentences). I don´t dare to explain right now, in a theoretycal approach, how to use it. Hope it was useful for you!

Exactly, when you are talking in plural, you use the word "S", for example: "these books are mine" in Spanish is "Esos libros son mioS". So, you must add the letter "S" because you are speaking in plural