Look for "trigger words." If the sentence contains ayer (yesterday), you are likely using the Preterite. If it says siempre (always), look toward the Imperfect or Present tense.
This exercise often requires you to rewrite sentences using pronouns to avoid repetition. Juan compra las flores. → Juan las compra. p137 estructura 2 actividades practice it
Using direct and indirect object pronouns (me, te, lo/la, le, etc.) in sequence. Look for "trigger words
Page 137 (P137) usually marks a transition in Spanish learning modules toward more complex sentence structures. In most standard curricula, on this page focuses on one of three key pillars: Preterite vs. Imperfect: Navigating the two past tenses. p137 estructura 2 actividades practice it