As you will see below the preterite suffers from an odd transformation. Even native speakers make the mistake of conjugating the preterite of traducir wrong. For instance instead of traduje they mistakenly say traducí.
Present:
Preterite:
The reason traces back to the Latin forms (and possibly farther back than that). In Latin, duco/ducere "to lead" changes "c" to "x" (c+s = x) to form the perfect tense: duxi, duxisti, duxit, ... This verb is the root of traducir, conducir, etc. Latin "x" normally corresponds with Spanish "j", hence conduje, condujiste, condujo, ....
Although most Spanish verbs are more regular, a few common irregular verbs still preserve this direct derivation of Latin perfect tense conjugations with unaccented personal endings -e, -iste, -o, -imos, -ieron, such as querer>quise, tener>tuve, estar>estuve, traer>traje, etc.
"Traducir" is an irregular verb that follows the conjugation model of "conducir". In Spanish there are a few irregular verbs, some of them are completely irregular and others just partially.
At the conjugation site of Instituto de Verbología Hispánica you can find the 101 conjugation models in Spanish with the list of irregulars; and this data base allows you to find any infinitive and indicates all the information about the verb you're looking for.
