Use the preterite for completed past actions — events with a clear beginning and end, a specific number of occurrences, or actions that interrupt ongoing situations.

Signal words: ayerel lunes pasadode repenteuna vezdos vecesen 2010anocheel año pasado
Pronoun Preterite
yo derribé
derribaste
él/ella/Ud. derribó
nosotros derribamos
vosotros derribasteis
ellos/ellas/Uds. derribaron

Use the imperfect for ongoing or habitual past situations — background description, states of being, repeated actions without a defined endpoint, and setting the scene.

Signal words: siemprenormalmentea vecestodos los díascuando era niñode niñomientrasen aquel entonces
Pronoun Imperfect
yo derribaba
derribabas
él/ella/Ud. derribaba
nosotros derribábamos
vosotros derribabais
ellos/ellas/Uds. derribaban
Key contrast

A classic contrast: "Mientras comía (imperfect — ongoing), sonó el teléfono (preterite — interrupting event)." The imperfect sets the scene; the preterite delivers the action.

Preterite vs Imperfect: common questions

How do I know when to use preterite vs imperfect in Spanish?

Ask whether the action was completed (preterite) or ongoing/habitual (imperfect). If it has a clear end point, use preterite. If it was a background state or repeated routine, use imperfect.

Can both tenses appear in the same sentence?

Yes — this is common. The imperfect provides background context ("I was walking") and the preterite delivers the main event ("when I saw him"). "Caminaba por el parque cuando lo vi."

Do verbs like saber and conocer change meaning between preterite and imperfect?

Yes. Saber in the preterite means "found out"; in the imperfect it means "knew". Conocer in the preterite means "met (for the first time)"; in the imperfect it means "knew/was acquainted with".

Practise both tenses with derribar using spaced repetition.