Nouns: Cases: Prepositional

The prepositional case (in English, the "object case" is used here) follows (some) prepositions and basically names the place where actions occur. (Thus, an alternate name some resources may use is locative case.)

Prepositional case answers the questions: о ком? о чëм? где? about whom? about what? where?
In Russian grammar books, when there is a table of case endings, prepositional is always shown with O, because this case must follow a preposition.

(See here for discussion of prepositions)

Prepositional case is introduced early in our book because it's so easy. There are only two* endings, И and Е, and the rules for which are simple and as close to being without exceptions as Russian gets. There's no consideration of animacy and only ten nouns where gender matters; it's all about the way the noun ends.

1. Does it end in ИЙ, ИЕ, or ИЯ? Then replace the final letter with И:

санаторий о санатории about the clinic
здание о здании about the building
армия об армии about the army

2. Does it end in Ь? Then replace the Ь with И:

ночь о ночи about the night
день о дне about the day
мать о матери about a mother

(Note: the -EP- infix shows up in all oblique cases (i.e., all but nominative singular) of both мать and дочь (so o дочери).)

3. Is it a neuter noun ending in МЯ? (This is the trickiest, but you'll get used to these nouns and their infix.) Then replace the Я with ЕНИ.

имя об имени about the name

4. Anything else? Replace any final vowel with E and add E to final consonants. Two spelling notes: Й plus E is just E, so the Й vanishes, and yes, words ending in E have the same form as their nominatives.

брат о брате about the brother
чай о чае about tea
герой o герое about a hero
папа о папе about Dad
сад o садe about the garden
окно об окне about a window
море о море about the sea
вода о воде about water

* Okay, actually there are three. (You didn't really think it was going to be that easy, did you?) Due to an historical quirk, some masculine (mostly) one-syllable nouns (or compounds with them) have a different ending in the locative (which is why O is used in grammar tables and charts). These nouns are -E with о, при, and по, about, under (of time), and along. But with в and на, in and on, they have the ending -У:

сад garden o садe but в саду in the garden
день day о дне but на дню on the day
порт port o портe but в порту in the port
край edge  о крае but на краю at the edge
берег shore  о береге but на берегу on the shore
аэропорт airport o аэропортe but в аэропорту at the airport