Nominative

Prepositional

Masculine

1. Add : “е”.
2: Some nouns (mostly single syllable) take “у” (but not after preposition “о”)

Завод (factory)

Инженер (engineer)

Город (city)

На заводе (at the factory)

Это история об инженере (this story is about the engineer)

Мы живем в городе (we live in a city)

Feminine

1. Replace “а” with “е”.
2. Replace “я” with “е”.
3. Replace “ь” with “и”

Река (river)

Стена (wall)

Семья (family)

Крепость (fortress)

Мы рыбачим на реке (we are fishing at the river)

Картина висит на стене (The picture is hanging on a wall)

Это вопрос о семье (That’s a question about the family)

Войска находятся в крепости (the troops are in a fortress)

Neuter

1. Replace “о” with “е” 
2. “е” remains unchanged.

Вино (wine)

Стекло (glass)

Зеркало (mirror)

Он спросил о вине (he asked about wine)

Муха сидит на стекле (The fly is sitting on a glass)

Он увидел свое отражение в зеркале (He saw his reflection in a mirror)

Plurals

Look at the last letter nominative singular form (dictionary form) and...
1. Replace "а", "о" or a consonant with "ах".
2. Replace the last letter «ь» with "ях"

Туристы (tourists)

Гвозди (nails)

Страны (countries)

Саша пишет о туристах(Sasha is writing about the tourists)

Он выяснил о гвоздях (He made an inquire about the nails)

Она побывала в разных странах (she’s been in different countries)