"I wrote to them every summer."
Translation:Yo les escribí cada verano.
Sin contexto suena mucho mejor y más natural usar 'escribía. Yo leo la frase tal cual y me suena muy mal con 'escribí', pero habrá contextos en donde si se vea más natural.... Por decir algo, "yo les escribí una carta aquel verano" o "yo les escribí una carta en verano", .......Pero al ser durante un período de tiempo ('cada verano') me suena regular lo de "escribí", yo tb elegiría "escribía"
To put it another way:
WHAT you wrote takes the direct object.
TO WHOM you then sent the writing takes the INdirect object.
(uinni, you were both correct and clear. I just thought this little example of mine might put the direct and indirect options in contrast to one another in a way that would clarify usage for those not accustomed to parsing sentences.)
Because the indirect object (me, te, le, nos, les) is never optional in Spanish. Even if you spell out who is indicated by les, you still have to include les.
Your sentence should have read Yo les escribí a ellos cada verano.
(In fact, sustained action over time usually takes the past imperfect: Yo les escribía a ellos cado verano. But some who know Spanish better than I say usage of past perfect v. past imperfect isn't as rigid as we are taught in school. They say the past perfect or preterite is a correct option in your sentence. Even so, you still have to include the indirect object pronoun.)