The five jobs of -ся verbs

English has no single counterpart to -ся. Depending on the verb, it corresponds to "oneself", to "each other", to a passive, to an intransitive verb, or to nothing at all. Sorting -ся verbs by what the ending does helps you translate them and, just as important, pick the right case after them.

Passive -ся is mostly imperfective and mostly third person: Мост строится, Билеты продаются в кассе ("Tickets are sold at the box office"). For a finished result Russian switches to the short participle: Мост построен.

Middle -ся describes what a thing does on its own or by its nature, where English uses a plain intransitive verb: Стекло бьётся, "Glass breaks". The classic trap is Собака кусается. It does not mean "the dog is being bitten" but "the dog bites": a warning about the dog's character, the kind you see on a dacha gate.

Emotion verbs are where -ся is simply part of the word: бояться and смеяться have no form without it. Радоваться does have a partner, радовать ("to make glad"), but the meaning shifts.

The case rule: in standard Russian a -ся verb does not take a direct object in the accusative. Verbs of feeling that have an object use the genitive or the dative: бояться собаки, радоваться весне, стесняться родителей. In casual speech you will hear бояться бабушку or слушаться маму; dictionaries mark that as colloquial, so keep it out of writing.

English speakers mostly go wrong by dropping -ся where English has no "oneself". Я учу on its own means "I teach"; "I study at university" is Я учусь. Фильм начинается в восемь, "The film starts at eight": the film is not starting anything, so it needs -ся.