Questions about Pronominal Verbs
Q: Why is it yo me acuerdo and NOT yo acuerdo?
R: Acordarse is a reflexive verb, while recordar is not.
Q: Why is it tú te bañas and NOT tú se bañas?
R: Remember that the reflexive pronoun (te) must agree with (i.e. refer to the same person as) the subject of the verb (tú).
Q: Why is it ellos se lavan las manos and NOT ellos llavan sus manos?
R: With body parts, use a pronominal verb, such as lavarse. Note also that you should use a determiner, las, rather than a possessive, sus.
Q: Why is it yo me enojo and NOT yo enojo to say "I get angry"?
R: The verb enojarse is reflexive and so must appear with a reflexive pronoun that agrees with the subject. Enojar and other reflexive verbs can be used non-refelxively if they appear with an object, e.g.: Yo enojo a mi hermano. ("I make my brother angry.")
Q: Why is it Me arrepiento de haber ido ahí and NOT Arrepiento de haber ido ahí?
R: This is simply one of those verbs that is always reflexive and so it has to be used with a reflexive pronoun.