Cuando comenzar) a escribir la novela mi madre ( ceer) que era una novela de amor yo le explicar) que se trataba de una novela policiaca.

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.

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.