We are going to look at “direct object pronouns” here, which are los complementos directos in Spanish. You may be familiar with them already. First, we need to review some sentence structure specifics. In the most simple of sentences, there are two parts: the subject and the predicate (this includes the verb and other information).
The subject is: the person or thing doing the action in the sentence
The predicate is: the action, or verb phrase in the sentence.
For example: She reads.
The object is: the person or thing receiving the action. There are direct objects and indirect objects - we will examine direct objects only.
EG: She reads the newspaper.
Often, the predicate (or verb) can be followed by an object. These objects can be things or people and they “receive the action” of the verb. Objects may be replaced with direct object pronouns to avoid repetition, as we do in English.
For example:
I pick up the book. I look at the book and buy the book.
I pick up the book. I look at it and buy it.
In Spanish - we have many different options for our pronouns because we have to think of number (singular/plural) and gender (masculine/feminine).
To figure out which direct object pronoun to use when it is a thing:
  1. What is the thing “receiving the action”? For example: Luis lee el libro.
  2. Is there one thing or many? One: el libro.
  3. Is it masculine or feminine? Masculine: el libro.
  4. Decide which pronoun to use from the following table.
  Masculine Feminine
Singular lo la
Plural los las

Normally we put the direct object pronoun in between the subject (Luis) and the verb (lee). Our sentence would change from:
Luis lee el libro → Luis lo lee.
A good way to figure out what the object in the sentence is to rephrase it as a question.
Luis lee el libro.
¿Qué lee Luis? Un libro.
Luis lo lee.
Paloma saca la foto.
¿Qué saca Paloma? La foto
Paloma la saca.
OJO: When the sentence is in the present progressive the direct object pronoun is either placed before the verb, just like you learned before when there is only one verb in the sentence; or attached to the end of the second verb, when it is an -ing verb (-ando or -iendo).
Marisela está cantando la canción.
Marisela la está cantando / Marisela está cantándola.
The same thing happens when we have two-verb combinations; the pronoun can be put before the first verb or attached to the end of the second verb, when it is an infinitive.
Las chicas quieren llevar las tortillas.
Las chicas las quieren llevar / Las chicas quieren llevarlas.
To figure out which direct object pronoun to use when it is a person:
  1. Who is the person “receiving the action”? For example: Visito a mi abuelo. Mi abuelo is the direct object because he is what (who) I “visit”.
  2. Is there one person or are there many? One: mi abuelo.
  3. Is the person male or female? Male: mi abuelo.
  4. Decide which pronoun to use from the following table.
 yo me
 tú te
 él lo
 ella la
 usted lo / la
nosotros/as nos
vosotros/as os
ellos los
ellas las
ustedes los / las

Our sentence would change from:
Visito a mi abuelo → Lo visito.
Let’s look at some more examples where people are the direct objects:
 Conozco a ti → Te conozco. 
 Ella ama a él → Ella lo ama. 
 Diego ve a Ana → Diego la ve. 
 Ellos llaman a nosotros → Ellos nos llaman. 

OJO: In Spanish, when people are the direct objects of verbs, we need to place an “a” in front of them. We refer to this “a” as the personal a since it’s used when people are direct objects. The personal a is only used when the direct object is a person.
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