A preterite verb is a past tense verb that has a definite beginning or end.
Example: I ate. Comí. (Has an end)
I met John. Conocí a Juan. (Has a definite beginning.)
There are 3 irregular preterites that you must memorize. (ir and ser, which are alike in the preterite) and dar, which has an er/ir preterite ending. None of these preterites have accents.
There are severa; preterites that have irregular stems: andar/anduv, caber/cup, decir/dij, estar/estuv, hacer/hic, poder/pud, poner/pus, querer/quis, saber/sup, tener/tuv, traer/traj, and venir/vin. These have unaccented, irregular endings: e,iste, o, imos, ieron (except for eron for traj, and dij).
Finally, there are irregular spelling changes. One is hico becomes hizo, to preserve the s sound of hacer.
Another spelling change is due to the fact that you cannot have 3 vowels together in Spanish unless the middle i is accented. If it is not, you must change the i to y.
Example: He heard. Oió is impossible, Oyó is O.K.
They built. Constuieron is wrong, Construyeron is O.K.
Finally, ir stem-changing verbs have a stem-change in the third person singular and third person plural form. In this case, the o of ir stem-changers becomes u, and the e of ir stem-changers becomes i.
Example: He slept. She slept. You (usted) slept. Durmió. They slept. You all slept. Durmieron.
He/She/You served. Sirvió. They/Y'all served. Sirvieron.
Pick the right preterite stem, the right preterite conjugation, etc.