Transitive and intransitive constructions: use of "se" (by Lazarus)
The most conservative classifications give SE 6 different uses; the most standard ones 7-8, and other more detail ones over 10 uses. More or less:
1) When you get any the 8 combinations 'le(s) lo(s)' or 'le(s) la(s)', with indirect and direct object pronouns before the verb, 'le(s)' changes to SE, always.
2) SE can be a reflexive direct object pronoun: Yo me lavo y él se lave (I wash myself and he washes himself). It can also be a reflexive indirect object: Te limpio el zapato y se limpia el zapato (... he washes his own shoe)
3) SE can be a reciprocal pronoun: Se aman (they love each other).
4) SE can be used for impersonal constructions. The verb is always in 3rd person singular, and there is no grammatical subject, but there can be objects: Se recibe a los invitados.
5) SE can be used for 'passive reflexive' constructions. The verb can only be in 3rd person, and it must agree in number with the subject, which is normally placed after it: Se vende esta casa. Se venden estas casas. (these houses are sold)
6) SE is sometimes an essential part of some verbs that cannot function without it, and does not even appear in the dictionary without the pronoun: Me arrepiento.
7) With many transitive verbs, it makes them intransitive, often eliminating the cause or agent that provoked the action: Rompo la radio (I break the radio). La radio se rompió (the radio broke). Lo enfadé (I annoyed him). Me enfadé (I got annoyed).
8) With some intransitive verbs, such as 'ir' or 'caer', it places special focus on the beginning of the action, or whatever caused the change. 'Ir' requires a destination, like 'to go (to a place)'; 'irse' doesn't, like 'to leave'.
9) Many transitive verbs take a SE that delimits the action, sometimes modifying the meaning of the verb. Most well known examples are verbs of consumption, like 'comer', 'beber', 'fumar',... 'Comer' is one of the few ones that sometimes requires this 'SE'; in all other verbs it is similar to the difference between 'wash' and 'wash up', and suppressing them does no harm.= to make others worried Preocuparse = to worry
And now the explanation:
Many Spanish verbs are transitive, i.e. they need a direct object. For example:
' romper = to break (something) ' hundir = to sink (something) ' quemar = to burn (something)
These transitive verbs assume that there is something or someone (an object) upon which the action is applied, so if you say "el barco hundió" (the ship sunk), you cannot assume that the ship just couldn't float anymore, since the verb is transitive. The sentence is incomplete in Spanish because after hearing "The ship sunk", any native would be expecting an object, i.e. something to sink. So, the ship sunk... another ship perhaps? Without an object, the sentence sounds incomplete and makes no sense. Spanish's solution for this is add an extra "se" that turns the verb into intransitive, so no object is expected, and everything is assumed to be related to the subject. For example, "El barco se hundió" would be "The ship sunk", but this time, the "se" tells us that this ship is not sinking other ships or anything else, but it is just sinking, nothing more.
The verb "preocupar" requires someone to make worried (the object). You can make your parents worried (preocupar a tus madres), and things can make you worried (la situación me preocupa). However, if you are just worried, with no object (no one to make worried), then Spanish resorts to this "se".
Me preocupo Te preocupas Se preocupa Nos preocupamos Os preocupáis Se preocupan
This "se" has no translation in English, since "worry" can be used as intransitive in English, but "preocupar" cannot without this pronoun.
As Eddy mentioned, this "se" is generally called "pronominal", and more intuitively, intransitivizer "se" by other authors.