Direct objects pronouns

The object that directly receives the action of the verb is called the direct object.

Bill hit the ball.
"Ball" receives the action of the verb "hit."

Sherry reads the book.
"Book" receives the action of the verb "reads."

When the pronoun replaces the name of the direct object, use the following pronouns:

me (me)
te (you-familiar)
lo, la (him, her, it, you-formal)

nos (us)
os (you-all-familiar)
los, las (them, you-all-formal)

Now, some examples where the direct object is a person.

I know you.                             te  = you
Te conozco.      

She loves him.                      lo =  him  
Ella lo ama.

She loves me.                       me = Me
Ella me ama.

Juan sees her.                      la = her
Juan la ve.

They call us.                          nos = us
Ellos nos llaman.

We call them.                       los = them
Los llamamos.

 In a negative sentence with one verb, the direct object pronoun is placed between the negative word and the conjugated verb.

Affirmative Sentence

I buy the books.
Compro los libros.
Los compro. (I buy them.)

Negative Sentence

I don't buy the books.
No compro los libros.
No los compro. (I don't buy them.)

In sentences with two verbs, there are two options regarding the placement of the direct object pronoun.

1. Place it immediately before the conjugated verb.
2. Attach it directly to the infinitive.

 Lo quiero ver.                       Quiero verlo.
I want to see it.                        I want to see it.

These same rules apply for questions and negative statements.

¿Lo debemos comprar?
¿Debemos comprarlo?
Should we buy it?

Juan no lo necesita lavar.
Juan no necesita lavarlo.
John doesn't need to wash it.