Spanish stem changing verbs c>j in the simple pastThere is a specific irregularity in El Pretérito Indefinido in Spanish with verbs that end in -cir. They change the c to a j in all forms. On top of that, these verbs don´t have the regular -ir verbs endings but they share the same endings as the "Pretéritos graves".(except for the ellos form which is -eron, not -ieron.)
Have a look at these examples:
Conducir -> Maribel condujo con prudencia (Maribel drove carefully)
Decir -> Me dijeron que la fiesta estaba cancelada.
If you want to know more about the "Pretéritos graves" see Irregulares en Indefinido/mismas terminaciones