"Leí la carta que él le escribió a su familia". Si tu divides la frase en 2 partes, no tendría sentido. La estructura de una oración esta compuesta por: "Sujeto, verbo y predicado"

Yes, but the main verb is "leer", and the main subject is "la carta", so the purpose or command of the sentence is "lee la carta". "que le escribe a su familia" is really a descriptive phrase that explains which letter to read.

This is a subordinated sentence. The conjunction "que" is working as a subordinating one. Therefore the part where it divides is starting from "que". Principal part: Lee la carta Subordinated part: que le escribe a su familia

Lea la carta que escribió a su familia. Read in this sentece give a order. Wrote is a past simple, so the verb in spanish " escribir" in past is " escribió" because the subject is the third person of singular. The sentence is divided in 2 parts. One: Lea la carta. Two: Él escribió una carta a su familia.