I don't understand "mi", "tu", "su", and the other possessives. they don't make any sense to me. would someone mind explaining this concept to me, please?
Mi = my
Tu - your
Su = your, his, hers, their
Mis, tus, and sus are the plurals of those, used with plural nouns.
Those ones are really easy. Mi coche=my car, tu casa = your house. Such can mean his/her/ its/your/their... so sometimes you have to clarify by saying el coche due Roberto = Robert's house.
Also, remember that they must match the number of "things possessed" and not the number of "things possessing". So,
His car = su carro
Their car = su carro
His cars = sus carros
Their cars = sus carros
I remember that at first this made less sense to me than the pronouns themselves. I had it backwards, always trying to make the pronoun agree with the number of "things possessing".