THE PRETERITE
The preterite is the tense for finished actions. Where the present says what you do, the preterite says what you did and, crucially, treats it as over: hablé (“I spoke”), comí (“I ate”), viví (“I lived”). It is the tense you need before you can tell anyone about yesterday.
The regular endings
Take the stem, as in the present, and attach the preterite endings. One thing makes this easier than it looks: the -er and -ir classes share a single set of endings, so there are only two patterns to learn, not three.
hablar → habl- + …
comer → com-, vivir → viv-
Note the written accents on the yo and él forms: hablé, habló. They are not decoration. They mark which syllable is stressed, and that is the only thing separating hablo (“I speak”) from habló (“he spoke”) — the same letters, a different tense and a different person.
Preterite or imperfect?
Spanish has two past tenses, and choosing between them is the real difficulty — not the endings. The preterite presents an action as a completed event. The imperfect presents it as ongoing, habitual, or as the backdrop something else happened against.
Ayer hablé con mi hermana.
Yesterday I spoke with my sister. One finished conversation, at a known time.
Hablaba con mi hermana cuando llegaste.
I was speaking with my sister when you arrived. Imperfect for the ongoing action; preterite (llegaste) for the event that interrupts it.
A useful test: if you could say “I was doing” or “I used to do”, you want the imperfect. If you could put a full stop after it and move on, you want the preterite.
The irregular stems
A small group of very common verbs takes a different stem in the preterite, and then all six forms follow from it predictably. Learning the stem is most of the work:
tener — tuv-
estar — estuv-
poder — pud-
hacer — hic-
These take their own endings — -e, -iste, -o, -imos, -isteis, -ieron — and, unlike the regular pattern, they carry no written accent. The stress falls on the stem instead: tuve, not “tuvé”. See tener for the full paradigm.
Two things that catch people out
First, ser and ir are identical in the preterite. Both give fui, fuiste, fue, fuimos, fuisteis, fueron, and only context tells you which verb you are looking at: fui médico (“I was a doctor”) versus fui a Madrid (“I went to Madrid”).
Second, stems ending in -j drop the i from the third-person plural. Decir gives dijeron, not “dijieron”; traer gives trajeron. Compare decir and traer, where the site marks every irregular form in this color.