When the direct object of a verb is a person or people, it is preceded by the preposition a – this ‘humanises’:

Juan llama a María – Juan calls María

¿A quién estás llamando? – Whom are you calling?

Visité a Miguel – I visited Miguel

in contrast:

Visité Bilbao – I visited Bilbao (no a)

It may also be used if the object is an animal with a personal connection, especially a pet:

Nosotros amamos a nuestro perro – We love our dog

in contrast, if no personal connection, no a:

Yo amo los perros – I love dogs

It can also be used to emphasise a personal connection or bond to something, especially a country:

José extraña mucho a España – José misses Spain a lot

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The personal ‘a’ is not used after the verbs tener, ser, or haber/hay:

Tengo dos hermanos – I have two brothers

Mi mejor amigo es Juan – My best friend is Juan

Hay cinco chicas – There are five girls

There are exceptions to this, e.g., when tener is used to mean ‘hold’:

Ella tenía a su nieta en los brazos – she had her granddaughter in her arms

It can also be omitted to ‘depersonalise’ references to people, e.g.:

Juan está buscando dos obreros – Juan is looking for two workers

However, it is used with these indefinite pronouns, which all stand for people:

alguien – somebody/someone/anyone); preguntemos a alguien – let’s ask someone

alguno – some (in reference to people)

nadie – nobody/no-one; no veo a nadie con quien trabajo – I do not see anybody I work with

ninguno – none (in reference to people)

quién – who/whom (ans in example above)

cualquiera – (in sense) anyone, anybody