Грамматика русского языка. Часть 6. Падежи. Глава 11. Предложный падеж, множественное число. Russian Grammar. Part 6. Cases. Unit 11. The Prepositional Plural.

The prеpositional plural endings arе needed if a plural noun is following the prepositions в (in, at) or нa (on, at).

The prepositional plural endings for nouns are the same for all genders. There are two possible endings (ax or яx) and to determine which one should be used, look at the last letter of the nominative singular.

The ending for prepositional plural nouns ending in a consonant, a or o in the nominative singular is: ax. Add this ending to nouns ending in a consonant. To nouns ending in a or o, remove the last letter of the nominative singular, then add ax, for example:

ресторан	Они обедают в ресторанах.	They have lunch in restaurants.
квартира	Они живут в квартирах.	They live in flats.
место	Они сидят на местах у окна.	They are sitting in seats by the window.

The ending for prepositional plural nouns which end in anything else in the nominative singular (i.e. й, ь, я, ия, e, иe) is яx.

Remove the last letter of the nominative singular and add яx, for example:

автомобиль	Водители ждут в автомобилях.	Drivers are waiting in (their) cars.
станция	Пассажиры ждут на станциях.	Passengers wait at stations.

Nouns which have irregular nominative plurals form their prepositional plural from the nominative plural (but the choice is still only between the endings ax, or яx), e.g.:

Nominative singular	Nominative	Prepositional plural
город	города	городах
друг	друзья	друзьях
ребёнок	дети	детях
стул	стулья	стульях
человек	люди	людях

The words мать, дочь, время and имя form their dative plurals as follows:
мать – матерях, дочь – дочерях, время – временах, имя – именах.