Answer: You're right. The two verbs, which mean "to laugh," mean basically the same thing. Although you will find some regional variations, reírse is the more common of the two. Thus, while reí would be understood to mean "I laughed," it would be more common to say me reí.
There are at least two cases where one form is required:
Reírse de: When followed by de, the reflexive form usually means "to make fun of" or "to laugh at":
- Me reía de mi hermano. I used to make fun of my brother.
- Se reirán de su falta de sofisticación computarizada. They will laugh at your lack of computer sophistication.
- Me hace reír. She makes me laugh.
- Austin Powers no me hizo reir más de una vez. Austin powers didn't make me laugh more than once.
Question: How should you translate "the boy laughs at the father"? Are both of these correct?
- El chico se ríe al papá.
- El chico se ríe del papá.
No, only the second one is correct.
There's no logical reason why reírse de is used to mean "to laugh at" rather than reírse a or even reírse en. That's just the way it is. This is one of those cases where you should learn the preposition along with the verb.
There are other verbs that combine with prepositions you might not expect. Following are a few of them; this list is far from complete and is meant as a sampling rather than a full reference:
- contar con, to count or depend on
- pensar en, to think about
- acabar con, to put an end to
- abalanzarse sobre, to rush at or to fall on
- vacilar en, to hesitate before
- abatirse sobre, to swoop down on
- interesarse en o interesarse por, to be interested in
- alegrarse de, to be happy about
- apostar por, to bet on
- encontrarse con, to meet or to run into
- chocar con, to run into
- coger de, to hold by