В университетском буфете можно купить кефир и йогурт.

Because в means in / at here, and when Russian uses в to talk about location, the noun normally goes into the prepositional case.

So:

• буфет = buffet, snack bar, cafeteria counter
• в буфете = in the buffet / at the buffet

The adjective has to match the noun:

• университетский буфет = university buffet
• в университетском буфете = in the university buffet

So both words change because they are part of the same prepositional phrase.

It is prepositional singular.

The base form is:

• университетский буфет

After в meaning location, it becomes:

• в университетском буфете

Why the endings?

• masculine/neuter adjective in the prepositional singular often ends in -ом / -ем
• masculine noun буфет becomes буфете

So:

• университетский → университетском
• буфет → буфете

This is just normal adjective-noun agreement in the prepositional case.

Not exactly.

In Russian, буфет often means a snack bar, canteen counter, or a small place where food and drinks are sold, especially inside a school, university, theater, or office building.

So университетский буфет is more like:

• a university snack bar
• a university cafeteria counter
• a campus food counter

It does not necessarily mean an all-you-can-eat buffet.

Because this is an impersonal construction.

Можно literally means something like:

• it is possible
• it is allowed
• one can

So можно купить is literally it is possible to buy.

In natural English, we usually translate that as:

• you can buy
• one can buy
• it is possible to buy

Russian often leaves out a personal subject in sentences like this.

Because можно is followed by an infinitive to express what is possible or allowed.

So the pattern is:

• можно + infinitive

Examples:

• можно купить = you can buy
• можно читать = you can read
• можно войти = you may enter

So купить stays in the infinitive because it depends on можно.

This is an aspect question.

• купить = perfective
• покупать = imperfective

In a sentence like this, купить presents the action as a complete act: to buy, successfully, as a whole event.

So можно купить кефир и йогурт means that kefir and yogurt are available for purchase.

If you used можно покупать, it would sound more like:

• it is allowed to buy
• it is possible to buy repeatedly / in general / as an ongoing activity

In signs, notices, and simple statements of availability, можно купить is very common.

They are the direct objects of купить, so they are in the accusative case.

But both nouns are:

• masculine
• singular
• inanimate

For inanimate masculine singular nouns, the accusative form is usually the same as the nominative form.

So:

• кефир → кефир
• йогурт → йогурт

That is why they look unchanged.

Yes. Russian word order is fairly flexible.

This sentence has a neutral, natural order:

• В университетском буфете можно купить кефир и йогурт.

But you could also say:

• Кефир и йогурт можно купить в университетском буфете.
• Можно купить кефир и йогурт в университетском буфете.

These versions are still grammatical, but the emphasis shifts a little.

• starting with В университетском буфете emphasizes the location
• starting with Кефир и йогурт emphasizes the items

It can correspond to all of those, depending on context.

Можно is broader than English can. It can express:

• possibility
• permission
• general availability

In this sentence, it most naturally means something like:

• you can buy kefir and yogurt at the university buffet
• kefir and yogurt are available at the university buffet

So it is less about someone’s personal ability and more about what is available or possible there.

In this sentence, университетском is the adjective university modifying буфете.

So:

• университетский буфет = university buffet

Because the whole phrase is in the prepositional case after в, it becomes:

• в университетском буфете

In English, we usually do not copy the case change. We just say:

• in the university buffet
• at the university cafeteria/snack bar

So университетском itself means university, but the whole phrase gives the meaning in/at the university buffet.