The "personal a" in Spanish

Direct object (person)

We use the personal a between the verb and the person, when the person is a direct object. And it doesn't translate in English.

Veo una película
Veo a mi madre

In both these cases we have:
Subject - ¿Who sees ...? - Me! I'm the subject
Direct Object - What do I see? Who do I see? - A movie! My mum! - Those are direct objects.

The personal a goes before a person that happens to be a direct object (but only if it's a person):

Entiendo el problema
No entiendo a Leonor

Recordamos el viaje
Recuerdas a tu primer novio

Pets?

This can also happen with some animals, specially pets that are close to us, that we're basically personifying them a bit in this case.

Veo los leones / Veo a los leones
With this, you can choose, and we hear both really, but... technically, it'd be the first.

Veo a mi perro or veo a Nobel (using the dog's name)
Here we definitely use it.

It's a bit subjective nowadays what animals are pets and which aren't sometimes, and some people have more of a tendency to personify animals (me!) so you can hear something like:

Llevaron a la tarántula al veterinario - They took the tarantula to the vet - Even though you don't need the "a" and most people don't consider it a pet.

He salvado a la rana - I've saved the frog (true story)
He salvado al pájaro - I've saved the bird ---see how a + el = al? Same as usual!

"People pronouns"

We also use it when it's a pronoun that refers to a person.
It makes sense, right? Nadie, alguien, todos (nobody, somebody, everybody) are also people.

No veo a nadie en la puerta - I see nobody at the door

¿Conoces a alguien de la empresa? - Do you know anybody in the company?

Sí, conozco a todos - Yes, I know all of them.

This aslso applies sometimes with alguno/a, algún, ninguno/a, but again, only when it refers to a person.

Sí, conozco a algunos - Yes, I know some (of them)
Sí, conozco a alguna persona/alguna gente - Yes, I know someone/some people

This also happens with the yo, tú, él... pronouns, but we don't often use those.
We do sometimes to emphasise or mark a contrast:

A mí me ascendieron y a él lo echaron - They promoted me and they fired him.

But most of the time they are subjects or they are used with a different preposition, and as we will see, we don't double up.

Nosotros vamos con él a votar - We go with him to vote.
Nosotros is a subject, as often these are, and before él we have another preposition (con, de, por, para, en, según...).

Questions

This is exactly the same so no new reason, but a structure to be aware of.
In questions, we have the persona a too, of course:

¿A quién visitaste en Amsterdam? - Who did you visit in Amsterdam?
Think of the answer: Visité a Pedro (I visited Pedro).

It would require a persona a because Pedro is the direct object.
Therefore, the question requires it too, and it goes at the beginning (because it goes before quien, the word representing the person)

Same here ¿A quién vas a llevar al aeropuerto? - Who did you take to the airport.

See how in both situations we translate it as "who" (direct object)

In the following sentence, we have the same and the reason it's different, it's an indirect object. But see how the translation is different too, it's to whom or you'll translate the preposition ("to").

Also, most times, you'll have an indirect object pronoun in third person (le/les) so it's even clearer.

¿A quién le diste el paraguas? - To whom did you give the umbrella? / Who did you give your umbrella to?

Reminder of the a with indirect objects

We just saw the a in a question where the person is an indirect object. Of course we recognise this a with indirect objects in statements, not only questions, and not only at the beginning.

Remember how at the beginning I said that the personal a doesn't get translated?
It's true, when the person it's a direct object. It also doesn't get translated in these examples with indirect objects ("me gusta" types being the most common case):

A nadie le gustan los insectos - Nobody likes insects
A alguien le apetece un trozo de tarta? - Someone fancies a piece of cake?
A mí no me saludaron - me, they didn't greet me (at the beginning for emphasis)

However, when the person is an indirect object, the a often gets translated.

No le dieron ningún premio a Jesús - They didn't give any award to Jesus.

Check out this blogpost on direct and indirect object if you need to refresh the basics or this one for lo/la/le specifically with people. They have practice with solutions too!

When not to use it

There are three situations where we don't use it (and doing it is wrong):

Tener/Haber/Ser

We don't use it with tener or haber (hay...) or ser:

Tengo dos hermanos - I have two brothers
Tengo amigos en Argentina - I have friends in Argentina

Hay veinte personas en clase - There are 20 people in class
Hay un hombre esperando - There's a man waiting

Somos cuatro en el taxi - We are four in the taxi
Es una buena persona - He/she is a good person

There are other prepositions

This one comes quite natural. If we have other prepositions after the verb, something that will either make sense in the translation or is required by the verb, then we don't double up. Only one preposition is allowed:

Necesito hablar con Ángela - I have to speak with Angela - I have the preposition con, so that's enough.

¿Estás pensando en Hugo otra vez? - Are you thinking of Hugo again?

Unidentifyable and unspecific

If the person is more a category than a specific person, if it's very generic and not refering to someone in particular, then we don't use it, like here:

Necesito un amigo - I need a friend. Not a particular one, just someone who could be a friend.
It's different from Necesito a mi amigo - I need my friend, and I have someone in mind.

Buscamos un cocinero - We're looking for a cook. The title, the job, not a particular one. Very common when a type of person is required somewhere, before they become "a real specific person".

Querríamos un médico/abogado... - we'd like a doctor/lawyer ...

Again, requiring of a specific "type of person", "function","job" ... so it's more the category than a person.
We don't require of Ana, for example, because she's Ana, we require of her because she can do a task.

Y ya está!!

¡Hasta pronto!