Místnost má kapacitu třiceti lidí.

A quick breakdown:

• Místnost = room, subject, nominative singular
• má = has, 3rd person singular of mít
• kapacitu = capacity, direct object, accusative singular
• třiceti lidí = of thirty people, a genitive phrase modifying kapacitu

So the structure is basically:

[subject] + [verb] + [object] + [genitive phrase]

In very literal English, it is something like:

The room has a capacity of thirty people.

Because kapacitu is in the accusative case, since it is the direct object of má.

• dictionary form: kapacita
• accusative singular: kapacitu

This is a very common pattern with feminine nouns ending in -a:

• škola → školu
• firma → firmu
• kapacita → kapacitu

So after má in this sentence, kapacitu is exactly what you should expect.

Because třiceti lidí depends on kapacita.

Czech expresses capacity of thirty people by putting the whole phrase thirty people into the genitive:

• kapacita čeho? = capacity of what?
• kapacita třiceti lidí

So:

• třicet lidí is the more basic form you might see in a sentence like Místnost pojme třicet lidí = The room fits thirty people
• but after kapacita, the numeral changes to třiceti

In other words:

• třicet lidí = thirty people
• kapacita třiceti lidí = capacity of thirty people

Because Czech often uses case endings where English uses a preposition.

English says:

• capacity of thirty people

Czech says:

• kapacita třiceti lidí

The meaning of English of is carried by the genitive case, not by a separate word.

This is very normal in Czech:

• sklenice vody = a glass of water
• konec filmu = the end of the film
• kapacita třiceti lidí = capacity of thirty people

So if you are looking for a word matching English of, very often the answer is: there is no separate word; the case ending does the job.

Because lidí is the genitive plural form, while lidé is nominative plural.

Here are the important forms:

• lidé = people, as a subject
• lidí = of people / people, in genitive contexts

Compare:

• Lidé čekají venku. = People are waiting outside.
• kapacita třiceti lidí = capacity of thirty people

So in your sentence, lidí is required because the phrase is in the genitive.

Not exactly.

• místnost is a general word for room or interior space
• pokoj often means a more ordinary room in a home, hotel, or building

In a sentence about capacity, místnost sounds more natural because it suggests a space being used for people, events, classes, meetings, and so on.

So:

• místnost fits well for a room with a stated capacity
• pokoj is possible in some contexts, but it often sounds less neutral here

Yes, it is natural and correct.

Czech often uses mít kapacitu in a fairly direct way, just like English have a capacity.

But there are also other natural ways to say something similar:

• Místnost pojme třicet lidí. = The room can hold thirty people.
• V místnosti je místo pro třicet lidí. = There is room for thirty people.
• Kapacita místnosti je třicet lidí. = The room’s capacity is thirty people.

So má kapacitu třiceti lidí is good, but it is not the only possible phrasing.

Yes, Czech word order is more flexible than English word order because case endings show each word’s role.

The neutral order here is:

• Místnost má kapacitu třiceti lidí.

You could change it for emphasis, for example:

• Kapacitu třiceti lidí má místnost.

That is grammatical, but it sounds marked or stylistically special. A learner should usually stick with the neutral version unless there is a reason to emphasize something.

So the basic advice is:

• use Místnost má kapacitu třiceti lidí as the normal, safe order

Because some Czech numerals decline, especially in formal standard Czech.

For třicet:

• nominative/accusative: třicet
• oblique form used here: třiceti

So:

• Vidím třicet lidí. = I see thirty people.
• kapacita třiceti lidí = capacity of thirty people

For learners, the most useful thing is to remember the pattern, not every detail at once:

• after kapacita, use the genitive phrase
• that gives you třiceti lidí

This is one of the hardest sounds in Czech for English speakers.

A few helpful points:

• Stress in Czech is normally on the first syllable: TŘI-ce-ti
• ř has no exact English equivalent
• In careful speech, it is a special Czech sound somewhat like a mix of r and zh

A rough learner-friendly approximation is to aim for something like:

• trzhi-tse-ti or trrhi-tse-ti

That is not perfect, but it can help at first.

The most important thing is not to worry if your ř is imperfect early on. Czech learners usually improve it gradually with listening and repetition.