Spanish past tense irregulars
Everything that is not ar, eir, and ir, are called root or stem. in this case they change, this is why they are called Spanish Past stem-changing irregular verbs. Here you have the charts of the most common irregular verbs in the past.
Here are the charts with the changes. And click here to have the notes of the explanation.
This is a video explanation about this topic: Here is the class that is used to explain it.