English has many contractions--cannot/can't, has not/hasn't, it is/it's, etc.--the most common ones involving verbs and another element. An apostrophe is used to mark the spot where letters and sounds have been omitted. Too, English offers a choice: one may contract or not.

Spanish, on the other hand, has only two contractions, both involving an definite article and a preposition. When the appropriate context is present, one must contract.

THE RULE
When prepositions a and de are followed by a masculine singular definite article el, the contractions al and del, result; examples:

Voy al centro con la hermana del chico.

EXCEPTION
The contraction does not occur if the article is part of a place name¹ (and therefore capitalized) or the first word of a title²:

Voy a El Salvador¹. / Voy a leer un capítulo de «El amor en los tiempos del cólera».²

NON-EXCEPTION
The above does not apply to the personal pronoun él:

Este libro es de ella, no de él.