In Spanish, there are two verb forms used when speaking of the past. The preterit is the first of the two forms that you will learn.

For the regular -ar verbs, using comprar as an example, the preterit is constructed by adding the following endings to the verb stem:
compr + é = I bought
compr + aste = you bought
compr + ó = you (form.)/he/she bought
compr + amos = we bought
compr + asteis = you (fam. pl.) bought
compr + aron = you (form. pl.)/they bought

For the regular -er verbs, using aprender as an example, the preterit is constructed as follows:
aprend + í = I learned
aprend + iste = you learned
aprend + ó = you (form.)/he/she learned
aprend + imos = we learned
aprend + isteis = you (fam. pl.) learned
aprend + ieron = you (form. pl.)/they learned

And finally for the regular -ir verbs, using asistir as an example, the preterit is constructed as:
asist + í = I attended
asist + iste = you attended
asist + ó = you (form.)/he/she attended
asist + imos = we attended
asist + isteis = you (fam. pl.) attended
asist + ieron = you (form. pl.)/they attended

The preterits for ser ir and dar are irregular.

The preterit of stem-changing verbs that end in -ar or -er have no change in the preterit. However, those that end in -ir have a change in the third person singular and the third person plural. If the changing vowel in the stem is an e, it becomes and i in the third person singular and plural, if the changing vowel is an o, it becomes a u.

Sometimes the preterit form of common verbs are irregular and must be memorized.