reir vs reirse
¿Cuál es la diferencia entre reír y reírse? ¿Se puede usar cualquier?
3 Answers
The differences are very hard to explain, and if I try to give rules to memorize, things can get even harder. This is the sort of subtle (maybe not so subtle) difference where one can show true mastery of the language.
Anyway, "reír" simply states the process, providing no clues about the transition between a non-laughing state and a laughing one. "Reírse", on the other hand, invites others to consider these transitions, their consequences, and the effect on the person who laughs. While "reír" is simply the act of laughing, "reírse" is often a taunt (laughing at...) induced by someone, a spontaneous or transient laugh, or it simply highlights how much the person enjoyed laughing. "Reírse" is more likely to be used when there are emotions involved.
When you are making fun of someone, only "reírse de..." is used, for there is more than just the neutral act of enjoying laughing. With causative sentences, where people are made to laugh, the plain "reír" is the most common form: "Me hizo reír". There are not only regional differences sometimes, but personal ones. The whole idea is to convey your thoughts using more subtleties.
The pronominal form "reírse" is not used with another pronominal verb. Thus, "Me eché a reír" is common, but "Me eché a reírme" is almost unheard of. In sentences like "Vas a reírte cuando te lo cuente", it is very unlikely to hear a version without that "te".
This issues are still being studied. It is not easy at all. I wouldn't advice non-advanced students to worry too much about this.
The first verb reír means "to laugh".
The second verb reírse is reflexive and means "laugh at one's self".
Click here to see the reference article on reflexive verbs. 
Actually, I think both reír and reírse can mean to laugh without invoking the reflexive "at yourself" rule. Rio de ella, me rio de ella. I actually think the latter is more used. But what do I know?
