Learn Hardcore Russian: В лесу растут деревья.

лесу is in the prepositional case (also called the locative) used to indicate where something is. Although most nouns in this case end in -е (e.g. в городе, в магазине), certain masculine place nouns like лес, сад, and поле take a special locative ending -у, giving в лесу for “in the forest/woods.”

Why is the preposition в used here instead of на?

Both в and на can translate as “in” or “on,” but their usage is lexical. в is used with enclosed or bounded spaces (e.g. в комнате, в лесу, в школе), while на is used with open areas or surfaces (e.g. на лугу, на улице, на столе).

Why is there no article before деревья?

Russian does not have definite or indefinite articles like “the” or “a.” The noun деревья by itself can mean either “trees” in a generic sense or specific trees, depending on the context.

Why is деревья in the nominative case?

деревья is the subject of the sentence—it’s the thing doing the action (growing). The nominative case is used for subjects in Russian.