El uso de “a” personal (objeto directo)

In English, there's no difference in the way the following two sentences are structured:
• I see the tree.
• I see Teresa.
But in the Spanish equivalent, there's an obvious difference:
Miro el árbol.
Miro a Teresa.
The difference is only a one-letter word — a — but it's an essential one that's important to learn. Known as the personal a, the short preposition is used to precede direct objects when those objects are people. Although a is usually translated as "to," the personal a normally isn't translated into English.


Yo visito a mi familia en la ciudad de Montreal.
Yo visito con mi familia la ciudad de Montreal, este fin de semana.

Explico a Carmen la lección.
Veo a María todos los días.

• Esta noche estudio con Pedro.
• Todos los domingos, hablo por teléfono con mi familia.