In Spanish, there are only two contractions, del and al, and they are obligatory. Whenever you have the definite article el immediately following the preposition de, you must make the contraction del. Whenever you have the definite article el immediately following the preposition a, you must make the contraction al.

Es el dinero del señor González. (de + el)
Vamos al hotel. (a + el)

Note: "De" and "a" do not contract with any of the other articles.

Es el dinero de la señora García.
Es el dinero de los pasajeros.
Vamos a la oficina.
Vamos a las reuniones.

In Spanish, the word a immediately precedes the direct object of a sentence when the direct object refers to a specific person or persons. This personal a, which has no equivalent in English, forms contractions in the same manner as the preposition a.

Thomas llama a la secretaria.
Thomas llama al secretario.
Voy a ver a la señora Garcia.
Voy a ver al señora Garcia.

Examples of Del and Al

• Buenas noches. ¿Espera usted a alguien?
• Cerca de la ventana, por favor.