Learn Hardcore Russian: Я жду друга рядом с библиотекой. - I am waiting for a friend near the library.

Russian uses the verb ждать with a direct object and no preposition: you say ждать кого/что (“to wait for someone/something”), not “ждать для” or “ждать за.” So Я жду друга literally means “I wait friend,” which in English is “I’m waiting for a friend.”

Why is it друга and not друг?

Because the verb ждать requires an object in the genitive (traditionally) or accusative case. For masculine animate nouns like друг, the accusative form is identical to the genitive: друга. So you won’t see друг (nominative) after ждать here.

Is друга genitive or accusative in this sentence?

Formally, it could be analyzed as either, because they look the same for masculine animate nouns. Traditionally, ждать governs the genitive, but in modern Russian both genitive and accusative are used with nuances that aren’t important here. Practically: you just say ждать друга.

Why рядом с библиотекой and not some other ending on “library”?

The set phrase рядом с (“next to, beside”) requires the instrumental case: с библиотекой. That’s why it’s not библиотеке or библиотеку here.