The accusative case is used not only with direct objects (more about it here) but also with destinations. Destination is a place to which someone or something is going or being sent. It is usually used with verbs of motion like «идти» (to go on foot) or «ехать» (to go by transport).
And one of the common mistakes foreigners make, they use the prepositional case with the destination. Remember, the prepositional case (the ending -e) is used for location, when the person is already there, not when they are on their way. For the destination, it is used the accusative case. That means masculine, neuter or plural nouns are not changed. But feminine nouns have the endings -у / -ю. So this is why we say,
- Это Москва. Я живу в Москве. Я еду в Москву. – This is Moscow. I live in Moscow. I go to Moscow.
- Это Китай. Я живу в Китае. Я еду в Китай. – This is China.I live in China. I go to China.
- Это аптека. Я сейчас в аптеке. Я иду в аптеку. – This is a pharmacy. I am in the pharmacy. I’m walking to the pharmacy.
It is important to remember two question words:
- где? – where? (location) – prepositional case
- куда? – whereto? (direction) – accusative case
Asking these questions you always know what case of two to use.