In this article we want to offer you a summary of the different uses of the particle se in the Spanish language, since we have seen frequently that our students have a constant difficulty with this when they are studying Spanish as a second language.

In reflected passive sentence
It is constructed with the conjugated active verb form in the third person preceded by se. Reflected passive sentences don't use usually the agent complement. The nominal syntagm is subject nonpersonal of the sentence and agrees with the verb in person and number:
El ayuntamiento construyó las casas.
Se construyeron las casas.

In impersonal sentence
They don't have subject, that's why the verb always goes conjugated in the third person singular. The nominal syntagm that expresses the patient is personal direct object goes introduced by the preposition a:
La gente critica a los políticos.
Se critica a los políticos.

The reflected passive sentence and the impersonal form are two forms to omit the expression of the agent, who is no specified and does not receive a generic interpretation. But syntactically, these two kind of sentences have different structures. Reflected passive sentences can only be formed by transitive verb that accept the passive transformation. The impersonal form can be formed with any kind of verb.

Atonic personal pronoun in midconstructions
It is necessary to indicate the frequent use of midconstructions with an atonic personal pronoun based on indirect object; this pronoun establishes a special relation of belonging between the subject and the involved person.
Se nos ha roto el coche.

Some intransitive verb also appear in similar constructions:
Se le caen las cosas de las manos.

With pronominal verbs
They are verbs that are always constructed with se. A variant of this kind of verb without this form does not exist:
acordarse; arrepentirse; atreverse; olvidarse

In the sentences with pronominal verbs, in the reflective and the reciprocal ones, the pronoun sealternates with other pronouns of first and second person when the subject changes of person:
Me acuerdo. Te arrepientes. Nos atrevemos

With aspectual value
The variant with se adds an aspectual slight differences in meaning to the verbal action: totally concluded action or beginning of the action.
El atleta secome un pollo. / El atleta se fue a Atenas.

As a reflexive or reciprocal pronoun
When se has the pronoun function, takes the place of some verb's argument and has the direct object function. It appears with transitive verb when subject and direct object have the same referent.
El niño se lava. [reflective oration]
Los niños se pegan (unos a otros). [reciprocal oration]

Indirect complement of third person
Se is the form that takes the personal pronoun as indirect complement (dative), in singular (le) as in plural (les), when it precedes to some of the accusative forms lo, la, los, las:
El marido le dio un regalo a su mujer.
El marido se lo dio. [se = to his wife; lo = a present]