The Personal A

In Spanish, when a person is the object of the action of a verb, we have to put an a in front of the word or name. For example, in the sentence Paco sees Juan, Juan is the direct object of Paco's seeing, so we say Paco ve a Juan.

Spanish's flexible word order can result in sentences like:

• Juan ve a Paco todos los días.
  Juan sees Paco everyday.
  
  A Juan Paco lo ve todos los días.
  Paco sees Juan (the a tells us that it is Juan who is being seen) every day. The lo means him, a direct object pronoun used here to emphasize Juan and avoid ambiguity.
  
  Paco ve a Juan todos los días, pero Manuel no.
  Paco sees Juan every day, but he (Manuel) does not.
  
  Paco ve a Juan todos los días, pero a Manuel no.
  Paco sees Juan every day, but (he -- Paco -- does) not (see) Manuel.

Notice how the a indicates who is receiving the action of the verb, in this case "seeing". The presence of the personal a is more important than word order when determining who is receiving the action of the verb, and serves to avoid ambiguity with regards to the subject and object of the verb when both subject and object are capable of both doing and receiving the action of the verb:

• El perro muerde al hombre.
  The dog bites the man.
  
  El hombre muerde al perro.
  The man bites the dog.

This a is also used when referring to pets, but never when referring to inanimate objects:

• Paco ve a mi mascota.
  Paco sees my pet.
  
  Paco ve la casa.
  Paco sees the house.

The personal a is necessary in the first case: because the pet is capable of seeing Paco, without the a, we would be unsure of who was seeing whom. The first sentence without the a would be ambiguous, but it is unnecessary in the second case because no ambiguity is possible, i.e., the house cannot see Paco.

The personal a is not usually used with the verb tener:

• Tengo dos amigos.
  I have two friends.