The prepositional case (mostly the ending -е) is used to indicate locations:  

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The prepositional case is used after prepositions в (in) or на (on, at).

Я в банке (I'm at the bank)    Журнал на столе (Magazine is on the table)



If the word after the preposition “в” starts with в- or ф- and then a consonant, the letter -o should be added to the preposition: 

во Франции



Very often English speaking students have problems with finding the difference between "location" and "direction". This is very important because with locations, the prepositional case is used. And with directions, the accusative case is used. 

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In English, the preposition "to" (not "in") is used to indicate the direction. And also, verbs of motion (like "идти" or "ехать") will be good indicators that you should use the accusative case because it shows direction. 

  • Борис сейчас в банке - Boris is in the bank now,

  • Борис идёт в банк - Boris is going to the bank now.