-Ar and -er verbs do not stem change in the preterite. -ir verbs stem changes in the third person forms. The –ir verbs that stem change o-ue will stem change o-u in the preterite, 3rd person forms.
– ir verbs that stem change e-ie in the present tense, will stem change e-i in the preterite, third person forms.– ir verb that stem change e-i in the present tense, will also stem change e- in the preterite, third person forms

Divertirse

Yo me divertí
Tú te divertiste
Él, ella, usted se divirtió
Nosotros nos divertimos
Vosotros os divertisteis
Ellos, ellas, ustedes se divirtieron

To have fun

I had fun
You had fun
He, she, you -formal- had fun
We had fun
You- plural, Spain- had fun
They, You- plural- had fun

Dormirse

Yo me dormí
Tú te dormiste
Él, ella, usted se durmió
Nosotros nos dormimos
Vosotros os dormisteis
Ellos, ellas, Uds. se durmieron

To fall asleep

I fell asleep
You fell asleep
He, she, you-formal- fell asleep
We fell asleep
You all- Spain- fell asleep
They, You all fell asleep

Divertirse

Yo me divertí
Tú te divertiste
Él, ella, usted se divirtió
Nosotros nos divertimos
Vosotros os divertisteis
Ellos, ellas, ustedes se divirtieron

For example:
Yo me pongo cada vez que veo una injusticia.

Translation:
I get angry every time I see an injustice:

Using “ponerse”