There are many irregular forms of the preterite tense. Here are three of them:
Hacer = To do, to make
The verb hacer loses its accents for the yo and él/ella form. It also has a "z" for the él/ella form, so as to keep the same sound throughout the whole verb (kind of like the -car, gar, zar verbs)
Ser = to be and Ir = To go
The verbs ser and ir are conjugated exactly the same. You need the context of the sentence and all the other words used to know which meaning is being used.
Yo fui a Mexico. = I went to Mexico.
Yo fui un doctor. = I was a doctor.