Preposition and article contraction in Spanish - De, del, a, al

In Spanish, the prepositions "de" (from or of) and "a" (to) are contracted with the masculine definite article "el".

In a sentence, when we need to use "de" and "el", we contract it to "del". In the case of "a" and an "el", we contract it to "al".

Wrong:
Marta es la hija de el profesor (Marta is the daughter of the teacher.)

Correct:
Marta es la hija del profesor (Marta is the daughter of the teacher.)

Please note: It applies only to the masculine article "el" and not to the feminine "la" nor the plural "los" and "las". So we still say:

Voy a ir a la piscina por unos días. (I'm going to go to the pool for a few days.)

More examples:

Me voy al cine (I'm going to the cinema)
Esa guitarra es del maestro de música (That guitar belongs to the music teacher)
Rosaura llevó a sus hijos al parque (Rosaura took her children to the park.)