Breakdown of Compré veintiocho sellos en correos porque todavía me faltan cartas por enviar.
Questions & Answers about Compré veintiocho sellos en correos porque todavía me faltan cartas por enviar.
Because the sentence refers to a completed action in the past: the speaker bought the stamps, and that action is finished.
A native speaker is not describing a habit or ongoing background action here, so imperfect would not fit as well.
Compare:
- Compré veintiocho sellos = I bought twenty-eight stamps.
- Compraba sellos = I was buying / I used to buy stamps.
The accent mark shows both stress and distinguishes this form from another one.
- compré = I bought
- compre = present subjunctive form, as in Espero que compre sellos
In compré, the stress falls on the final syllable: com-PRÉ.