We use relative clauses to give additional information about something without starting another sentence. By combining sentences with a relative, your text becomes more fluent and you can avoid repeating certain words.
How to Form Relative Clauses
Imagine, a girl is talking to Tom. You want to know who she is and ask a friend whether he knows her. You could say:
A girl is talking to Tom. Do you know the girl?
That sounds rather complicated, doesn’t it? It would be easier with a relative clause: you put both pieces of information into one sentence. Start with the most important thing – you want to know who the girl is. Sigue leyendo
Su estructura es: 
Just a summary contrasting both expressions:
Para referirnos a lo que alguien dijo, podemos hacerlo de dos maneras: 1) Direct Speech: es decir, citando literalmente sus palabras (entre comillas) –> Mary said: “I work in this office”. 2) Reported Speech: “contando” lo que dijo, que es más normal –> Mary said (that) she worked in that office. (“That” aparece entre paréntesis porque podemos omitirlo) 