Learn Hardcore Russian: На вечеринке гости могут заказать напитки у официанта. - At the party guests can order drinks from the waiter.

На вечеринке is in the prepositional case after the preposition на with the meaning “at.” It tells us the location where the action happens (“at the party”).

What case is гости, and why is it used here?

Гости is nominative plural. It’s the subject of the sentence, the ones who perform the action (“the guests can order…”).

Why do we use the infinitive заказать after могут?

Могут is a modal verb meaning “can” or “may.” In Russian, modal verbs are followed by an infinitive to express ability or permission. Here it means “guests are able to order.”

What is the difference between заказывать and заказать?

Заказывать is imperfective: it describes a habitual or ongoing action (“to be ordering”).
Заказать is perfective: it describes a one-time or completed action (“to place an order”). In our sentence, perfective заказать implies the guests can place an order (at least once).

Why is напитки in the accusative plural?

Напитки is the direct object (“drinks”). Because it’s inanimate, its accusative plural form is the same as its nominative plural.

How does у официанта work, and why not use a dative?

У + Genitive here indicates the source or person who has something: “from the waiter.” You cannot use the dative (официанту) for “ordering from someone”; the dative would mean “to the waiter,” not “from.”

Can we change the word order in this sentence?

Yes. Russian word order is relatively flexible. You could say, for example:
Гости могут у официанта заказать напитки.
This shifts у официанта forward for slight emphasis on “from the waiter,” but the overall meaning stays the same.