The prepositional case has only one use: it appears after certain prepositions. It never appears without a preposition. Specifically it used to indicated:
The prepositional case is used after the prepositions в, and на when they indicate location:
| Молоко в холодильнике. | The milk is in the refrigerator. |
| Книга на столе. | The book is on the table. |
The prepositional case is used after the preposition на in combination with the verb играть when it indicates the musical instrument you play:
| Высоцкий играл на гитаре. | Vysotski played the guitar. |
| Я слушал, как буддистский монах играл на флейте. | I listened to a Buddhist monk playing the flute. |
The prepositional case is used after the preposition в when indicating the month in which something happened:
| Я родился в июле. | I was born in July. |
| Мы были в Москве в декабре. | We were in Moscow in December. |
The prepositional case is used after the preposition в when indicating the year in which something happened:
| Я родился в тысяча девятьсот восемьдесят девятом году. | I was born in 1989. |
| Мы были в Москве в две тысячи четвёртом году. | We were in Moscow in 2004. |
Note that году is an irregular preposition form used only when indicating the year in which something happened.
The prepositional case is used after the preposition о/об/обо when indicating the subject of an article, discussion, comment, book, etc.
| Мы поговорили о новых студентах. | We chatted about the new students. |
| Я прочитал статью об Америке. | I read an article about America. |
| Мы поговорили о том годе, который я провёл в России. | We talked about the year that I spent in Russia. |