Uses of the Spanish word "se"

This post is about the uses of the word "se" in Spanish.

As a reflexive pronoun of the third person singular and plural
In English, it's enough to say He takes a bath. However, in Spanish you say él se baña. Se acts as an reflexive pronoun. If you say to someone, él baña, he might ask back to you ¿a quién baña? as él baña is incomplete and needs an object.

Substitute for the indirect object "le" or "les" if a direct object (lo, la, los, las) is present in the sentence
Instead of saying le lo voy a dar, we say se lo voy a dar.

It's used when a part of a human body or clothing is involved in the sentence
Instead of using the possessive pronoun su, se is used and the part of the body or the clothing is accompanied by the definite article el or la.

He washes his hands
(Él) se lava las manos (instead of Él lava sus manos

She puts the necklace on
(Ella) se pone el collar (instead of Ella pone su collar)

He washes them (hands)
(Él) se las lava

She puts it on (necklace)
(Ella) se lo pone

Part of a pronominal verbs
Pronominal verbs are verbs that are accompanied by a reflexive pronoun. These verbs have to be memorized. Most of these are verbs of emotion.

Se fue - He went away
Se durmieron - They fell asleep
No te quejes - Don't complain
Se entristece - He feels sad
Se sorprendió - He got surprised

Reciprocal use
Se means each other

They love each other - ellos se quieren

Passive use
A sentence is in passive voice when the doer of the verb is not known. It's more or less has the same meaning of the French on.

En España se vive mejor que en México - The quality of life in Spain is better than in Mexico. (literally means, "One lives better in Spain than in Mexico")
Se fabrican un millón de móviles cada año - A million mobile phones are manufactured every year
Las manos se lavan antes de comer - One should wash his hands before eating
No se casa si no se tiene dinero - One should not marry if he has no money
Se prohibe fumar - No smoking
Se cree que la tierra es plana - It is believed that the earth is flat

Used with verbs such as olvidar to express that something was done or occurred unintentionally

Se me olvidó decirte que... - I forgot to tell you that... (literally means, "Telling you was forgotten to me")
Se me ocurrió una idea - I got an idea
Se nos hizo aburrido el viaje - The journey felt boring (to us)
Se me fue el dolor - It doesn't hurt anymore
Se le pasó el hambre - He's not hungry anymore
Se me quitó la mancha en el vestido - The stain on my dress got removed (by itself)
Se me ha vuelto a averiar el coche - My car has broken down again.