Answer: Cuando comencé a escribir la novela, mi madre creyó que era una novela de amor, yo le expliqué que se trataba de una novela policiaca.
Translation: When I started writing the novel, my mother thought it was a love novel, I explained her that it was a police novel.
Step-by-step explanation:
He describes a fact that happened only once in the past: I started writing the novel. That is preterite tense. Yo comencé.
He describes a fact that happened only once in the past: His mother thought it was a love novel. That is preterite tense. Ella creyó.
He describes a fact that happened only once in the past: I explained her that it was a police novel. That is preterite tense. Yo expliqué.