Learn Hardcore Russian: В клинике нет свободных кресел, поэтому я ждал на улице. - There are no free seats in the clinic, so I waited outside.

В клинике means “in the clinic.” The prepositional case is used after в when you talk about being located somewhere (a static location). The noun клиника (nom. sing.) changes to клинике in the prepositional case (prep. sing.), and you always use в + prep. to mean “inside” or “at” a place.

Why do we say нет свободных кресел instead of using есть or the nominative case?

Нет means “there is no / there are no,” so you use it when something is lacking.
After нет, the noun that follows must be in the genitive case (here кресел is gen. pl.).
The adjective свободные also takes the genitive plural ending -ых, becoming свободных.
Putting it together, нет свободных кресел literally means “there are no free seats.”

What does кресел refer to, and why is it in genitive plural?

Кресел is the genitive plural form of кресло (“armchair” or “seat”).

Genitive plural is required after нет.
Plural of кресло is кресла (nom. pl.), gen. pl. is кресел.
So свободных кресел = “of free armchairs/seats.”

What’s the difference between кресло and стул, and why might a clinic use кресла?

Кресло is an armchair or a comfortable seat (often with armrests).
Стул is a chair without armrests.
Clinics and waiting rooms often have cushioned or semi-comfortable seats called кресла, so Russian speakers would say свободных кресел rather than свободных стульев.

What does поэтому mean, and how is it used?

Поэтому is a conjunctive adverb meaning “therefore,” “so,” or “that’s why.” It connects the cause (no free seats) with the result (waiting outside). It always introduces the consequence: …нет свободных кресел, поэтому я ждал на улице.
“…there were no free seats, therefore I waited outside.”

Could we use потому что instead of поэтому here?

Yes, but they work differently:

Поэтому introduces the result: Cause, поэтому effect.
Потому что introduces the cause: Effect, потому что cause.
Example with потому что:
“Я ждал на улице, потому что в клинике не было свободных кресел.”

Why is the verb ждал imperfective, and what would подождал mean?

Ждал is the imperfective past tense (“I was waiting” or “I waited” in a general sense), focusing on the ongoing action.
Подождал is the perfective past tense, emphasizing that the waiting was completed (“I waited (and then stopped/went away)”).
Here the speaker likely wants to stress the continuous action, so they use ждал.

Why is ждал masculine, and how would a female speaker say it?

In Russian past tense, verbs agree with the subject’s gender and number:

Masculine singular: ждал
Feminine singular: ждала
Neuter singular: ждало (rare for people)
Plural: ждали
So a woman would say я ждала на улице.

Why is на улице used for “on the street” instead of another case?

The preposition на plus the prepositional case indicates location on or at an open surface.

Улица (nom. sing.) → на улице (prep. sing.) means “on the street.”
You use в
- prep only for enclosed locations (в парке, в доме), while на
  - prep covers open spaces (на площади, на вокзале, на улице).