Spanish - personal a

Personal 'a'
This is a concept that doesn't exist in English, so it can be a bit tricky to figure out.

Basically, why you would say:
Está tirando la pelota.

but then say:
Está tirando al niño.
Note: "al" is a word that is what is used when you have a + el together. a + el = al

It's all because of the concept of a personal 'a' in spanish.


Personal 'a'
If a person is a direct object of a verb, unless it's the verbs  'tener' or 'hay,' the spanish language adds an 'a' before the person. There is nothing like this in english. 

What is treated as 'a person?'
Actual people: 
A person can be Marta, girl, boy, my mother, your son. It can be used for objects that are unspecific or indefinite people, like doctor or carpenter, but it is usually only used in these cases if you KNOW the people.

Like, for 'I know two doctors' you would use an 'a' before 'mèdicos.'

ex.  Conozco a dos mèdicos.

'I need two doctors' - no 'a,' because you have no personal feeling toward these mèdicos.

ex. Necesito dos mèdicos. 

Pets:
You also use a personal 'a' before an animal if there is close feelings toward it, like a pet. You would not do this with a random animal, like a wild bird or random animal.
ex. Veo a mi gato, Batman. I see my cat, Batman.

Some pronouns are also treated like a person:
Alguien (somebody), nadie (nobody), quién (whom) require a personal 'a'. So do alguno (some) and ninguno (none) if they refer to people.
Ex. ¿A quién pertenece esta silla? Whose chair is this?
¿Médicos? No vi a ningunos. Doctors? I don't see any.
¿Carros? No vi ningunos. Cars? I don't see any.

Countries and objects:
A country or object can be personified, although it implies a personal relationship, such as great emotion. Like a country that an expatriate misses, or a favorite doll or toy.
ex. Yo extraño a los Estados Unidos. I miss the United States. (implies greater relationship or feeling)

Basically, personal 'a' denotes a personal aspect toward the object in question, is my impression.

Exceptions:
There is a personal 'a' used after tener if it is used to mean holding someone or having someone somewhere.
ex. Tengo a mi hermano en los brazos. I have my brother in my arms.
Tengo a mi hija en el pesebre. I have my daughter in the crib.

It can also be used when tener is used to indicate an exceptionally close relationship. Like, if one were talking about how you often feel alone and sad, but then you have your friends to help, you could say tener a mis amigos. But just saying I have friends would be, 'tengo amigos.'



My information came from these two sources:

The basics on this are here, although they miss a few things - http://www.studyspanish.com/lessons/persa.htm

More details, examples, and exceptions shown here - http://spanish.about.com/cs/grammar/a/personal_a.htm