The conjugations and accent mark placements in the preterite are the most difficult of all tenses because there are many spelling changes and irregulars. One tip is to remember that the rules you learned in the present tense apply to the present tense only: you have to learn a whole new set of rules! A key to memorizing and applying these rules is to categorize the verbs into either regular or irregulars. There are several subcategories for both regular and irregular verbs.
A. Regular Verb Endings
* Here’s your first piece of evidence that you need to memorize new rules for the preterite: The yo form does not end in ‘o’ (and for the past you will need an accent over the o) and the tú form does not end in ‘s.’
**In the present tense, -er verbs become 'emos' in nosotros and 'éis' in vosotros, but in the preterite, -ir and -er will conjugate the same way in nosotros and vosotros. Stay tuned for more differences!
Verbs such as hablar, vivir, beber, and comer are regular verbs and all conjugate as expected based on the model above.
B. Stem changing
In the preterite, -ar and -er verbs DO NOT stem change...Is it already getting old for me to state that this is yet another difference between the present and the past?
-Ir verbs that normally stem change in the present tense will stem change in the preterite tense, but they will stem change in different ways: one way is what letter they change to and the second way is the conjugations that are affected.
1.-Ir verbs that either stem change e-->i or e-->ie in the present will both stem change e--> i in the preterite. Verbs that stem change o-->ue in the present tense will only stem change o-->u.
2. Only the 3rd person singular (él, ella, usted) and 3rd person plural (ellos, ellas, ustedes) will stem change.
Examples: Preferir and Dormir
If you are frustrated and think this is a bizarre and arbitrary rule, just remember that spoken language developed before written language. Changes naturally occurred because they were easier to say. Say “prefierieron” five times fast and then say “prefirieron” five times fast and see which one is easier to say! If your Spanish pronunciation is more advanced, you'll even be able to tell that saying "prefirieron" is preferable (excuse the pun) to saying "preferieron."
Remember, the -ir verbs that do stem change will receive the same endings in the model for regular verbs.
C. Spell changing
1. -Car, gar, zar
Verbs that end in car, gar, or zar change spelling in the yo form only. Some examples are buscar, llegar, almorzar, and empezar. The spelling changes are made to reflect the desired pronunciation.
Examples: Buscar and Llegar
Note that all of these receive the same endings as all regular verbs.
As a reminder, -ar verbs do not stem-change in preterite. Almorzar will retain the o and empezar will retain the its second e.
2. -Eer, -aer, -oír, -oer, -uir
Verbs that end in -eer, -aer, -oír, -oer, -uir will change spelling in 3rd person singular (él, ella, usted) and 3rd person plural (ellos, ellas, ustedes). Some examples are creer, oír, construir and huir. The spelling and additional accent mark changes are also made to reflect the desired pronunciation. The spelling changes are:
While regular verbs only have accent marks over the ending for yo (í) and 3rd person singular (ó), some of the verbs in this category will get accents on the "i" in all forms except the 3rd person plural. In the 3rd personal plural, the "i" has already turned into a "y," making it "yeron." Some vowels are strong and some are weak, and sometimes two vowels in a row will form their own sound. When Spanish wants the weak vowel to stand its own ground, an accent mark is added. In this case, that vowel is an ‘i.’ Verbs that end in -eer,- oír and -aer, such as creer, leer, oír and caer, fit into this category. Verbs that end in -uir do not fall into this category, but they still receive an accent mark over the “í” in the yo form like is normally expected. You should consult your teacher about -uir verbs. In 2010, the RAE changed the accent mark pattern for the verb huir, but your teacher may have a preference for the old pattern.
Examples: Leer(additional accents and spelling change) and construir (spelling change only--accent marks are where expected)
II. Irregular Verbs
Irregulars do not have accent marks in the preterite, which is the main reason why it is important to know whether a verb can be classified as regular or irregular.
A. U,I,J verb endings
Some verbs can be classified as U,I, or J verbs. They are verbs that change their roots (the part of the verb that is not -ar, -er, or -ir) radically and adopt either a U, I, or J that they don’t normally have. One example is that tener changes to tuv before the conjugation ending is added, even though tener does not have a U in it. The new root is the same for all forms, including nosotros and vosotros. All of those verbs, regardless of whether they are originally -ar, -er, or -ir, will have the following endings attached to them:
* U and I verbs will end in ieron. J verbs will end in eron.
B. U Verbs
C. I Verbs
querer-quis venir-vin hacer-hic (hiz in 3rd person singular)
Examples: Venir and Hacer
A mnemonic device is: Mi amigo y yo fuimos a una fiesta. Él vino con el vino y yo traje el traje. My friend and I went to a party. He came with the wine and I brought the suit! No one ever said all mnemonic devices have to make sense.
D. J Verbs
decir- dij* traer-traj conducir-conduj
Remember that the endings for U,I,J are all the same except that the J verbs receive -eron as an ending in the 3rd person plural.
*Decir is both an I and a J root change.
The two following pairs of verbs are unique in their own ways. They should each be thought of as a unique pair.
Both ser and ir conjugate exactly the same way. You will know if someone means “I was” or “I went” based on context.
These two have their own set of irregular endings (dar is an -ar verb that gets -er/-ir endings and ver receives the -er/-ir endings for regular verbs, but with no accents)
Now you have your survival guide. Go forth and confidently navigate the past! Perhaps it is better stated: Go back and confidently conquer the past!
Erichsen, Gerald. "Strong Vowels and Weak Vowels." About.com. Web. 24 Dec 2013. http://spanish.about.com/od/writtenspanish/a/diphthongs.htm