В vs. на

Both "в" and "на" can mean "in, at". The choice of the preposition depends on the noun that goes after it. In most cases we use "в", but some nouns take "на". Here’re the most common of them:

На работе = at work

На улице = in the street

На заводе = at the factory

На почте = at the post office

На пляже = on the beach

Practice: try this exercise ( "в" vs. "на" + the prepositional case vs. the accusative case).