When to use "a" vs. "al"?

Question

For example, the sentence "Cambió a realismo social"

But, should it be "AL realismo social"?

How do you know when to use which?

Answer 1

A + el = al If you want to say "Cambió a el realismo social", then you would short it to "al". If you don't want the "el", then use "a".