Pasado libre son acciones completadas, cosas que ya hiciste en el pasado. El auxiliar para este verbo es "did" usado en preguntas y negarivas:
Did you eat chocolate?
Yes, I ate chocolate yesterday
No I didn't, I ate apples
Was she your girlfriend?
Yes, she was my girlfriend
Did you go to the William's party last Friday?
Yes, I did and it was the best party ever!
Did you do the math homework?
Of course I did, it was so easy!
Did he buy your soda in that oxxo?
Yes He did, and it taste very good