Do not teach the contrast as a keyword detector
Signal words can help beginners notice patterns, but they become a trap when students believe ayer automatically means preterite or siempre automatically means imperfect. The more useful question is how the speaker presents the past event.
The preterite presents an event or sequence with a boundary. The imperfect opens the scene from inside: description, background, habit or an action in progress. Students need to see that both forms can appear in the same story because they do different jobs.
Activity 1: background or event?
Show a simple scene with five facts. Some describe the situation and others move the story forward. Ask students to sort them into background and event before they conjugate anything.
Then turn the same scene into a two-column timeline. Students narrate the background with the imperfect and add completed events with the preterite. Because the conceptual choice comes first, the conjugation has a reason.
Activity 2: freeze the story and change the camera
Give a sentence such as Cuando llegué, Ana hablaba con el gerente. Ask what the camera is doing with each verb. Then change one form and discuss how the scene changes.
This works especially well with short image sequences. One student describes what was happening; the other introduces the event that interrupted or advanced the story. Swap roles so both students must use both perspectives.
Activity 3: build a story with two layers
The last retelling is the useful test. If students can narrate while choosing a perspective, they are doing more than completing a worksheet. Use their own story for correction, then ask them to retell one section after you change a detail.
- Layer one: weather, location, age, mood, habits and ongoing actions.
- Layer two: arrivals, discoveries, decisions, interruptions and completed actions.
- Final round: the student retells the story without seeing the tense labels.