Which tense - PRETERITE or IMPERFECT? 1

The fundamental distinction — The preterite views a past action as a completed event at a specific point: Ayer llegué tarde. The imperfect describes an action as ongoing, habitual, or background: Siempre llegaba tarde. The same action can be expressed in either tense — the tense tells us how the speaker views it.

Background vs interrupting action — A classic use: the imperfect sets the scene; the preterite interrupts it: Dormía cuando sonó el teléfono (I was sleeping [imp.] when the phone rang [pret.]). The imperfect action was already in progress; the preterite action broke into it. This pattern is very testable.

Time signal words as clues — Certain time expressions typically signal one tense: preterite — ayer, el lunes pasado, de repente, de pronto, una vez, por fin. Imperfect — siempre, todos los días, a menudo, normalmente, antes, cuando era niño/joven, generalmente. These aren't absolute rules, but they're reliable guides for choosing.