When is "al" not interchangeable with "a el"?

Question

This is a clear case where "al" cannot be replaced with "a el":> Al mirarlo, sonrió.Are there any other cases?

Answer 1

In the case you have mentioned above, the verb mirar is treated as a noun with the definate article el. The use of the prepoition a communicates something like "upon seeing" or "when seeing". Regardless, the instance of al is a contraction and cannot be seperated.

There are only two contractions in Spanish, al and del , and they are obligatory (unlike English contractions) whenever a and el are together, or de and el are together. The only exception to this is when the pronoun él (he) is being said instead of the article el (the).

examples:

Vieron al profesor.

Vieron a él.

Este libro es del profesor.

Este libro es de él.