If a sentence has both a subject and a direct object the subject is the actor and the direct object is acted upon. For example in the sentence He is reading a book, He is the subject, because He is doing the reading, while a book is the direct object, because it is the thing being read. It is acted upon by the subject.

Sentence	Finding the subject	Subject	Finding the object	Direct object
He is reading a book.	Who is reading?	He	What is he reading?	a book
Pablo treats Maria without respect.	Who is treating (without respect)?	Pablo	Who is Pablo treating (without respect)?	Maria

We already know the word order for the sentences above in Spanish.

He is reading a book.	Él lee un libro.
Pablo treats Maria without respect.	Pablo trata Maria sin respeto.

When we use a pronoun instead of a book or Maria, then the pronoun comes immediately before the verb.

He treats me with much respect.	Me trata con mucho respeto.
I believe you.	Te creo.
Are you reading the book? Yes. I am reading it.	¿Lees el libro? Sí. Lo leo.
Are you buying the house? No. I'm not buying it.	¿Compras la casa? No. No la compro.
Pablo treats her without respect.	Pablo la trata sin respeto.
They are taking us to the island.	Nos llevan a la isla.
We are taking you to the island.	Vos llevamos a la isla.
Don't you believe them?	¿No los crees?

Breakdown of ¿Escribe las cartas? Sí. Las escribo.