Dokumentární film je dlouhý.

Czech does not use articles like English does. So Dokumentární film je dlouhý can mean:

• A documentary film is long
• The documentary film is long
• Documentary film is long

Which one is meant depends on context.

If Czech speakers want to make it clearer, they can use words like ten (that/the) in some contexts:

• Ten dokumentární film je dlouhý. = That / the documentary film is long.

Je means is. It is the 3rd person singular form of the verb být (to be).

So:

• film = film
• je = is
• dlouhý = long

Very literally: Documentary film is long.

Because dlouhý has to agree with film.

In Czech, adjectives change their endings to match the noun’s:

• gender
• number
• case

Here, film is:

• masculine inanimate
• singular
• nominative

So the adjective must also be masculine singular nominative, which gives:

• dlouhý

Compare:

• dlouhý film = a long film
• dlouhá kniha = a long book
• dlouhé město would be grammatically neuter, though not a natural phrase in meaning

The same agreement also applies to dokumentární.

Everything here is in the nominative singular.

That is because film is the subject of the sentence:

• Dokumentární film = the subject
• je dlouhý = what is being said about the subject

So:

• film → nominative singular
• dokumentární → nominative singular masculine form
• dlouhý → nominative singular masculine form

In Czech, adjectives usually come before the noun they describe, just like in English.

So:

• dokumentární film = documentary film
• dlouhý film = long film

That part is very straightforward for English speakers.

But notice the difference between:

• Dokumentární film je dlouhý. = The documentary film is long.
• Film je dokumentární. = The film is documentary.

In the first sentence, dokumentární directly describes the noun film.
In the second, it is part of the predicate after je.

It can mean either, depending on context.

With film, it normally means long in duration. So here it means:

• The documentary film is long
• in other words, it lasts a long time

If you use dlouhý with other nouns, it can mean physical length:

• dlouhý stůl = a long table
• dlouhá ulice = a long street

So the exact sense comes from the noun and the situation.

A helpful rough guide for an English speaker is:

• Dokumentární ≈ do-ku-men-TAAR-nee
• film = film
• je ≈ yeh
• dlouhý ≈ dloh-hee

A few useful points:

• Czech stress is usually on the first syllable of each word.
• á and ý are long vowels.
• je is pronounced like ye in yes, not like English jee.
• ou in dlouhý is a diphthong, roughly like oh-oo blended together.

The accent marks show vowel length.

So:

• á = a long a
• ý = a long ee-like sound

In Czech, vowel length matters and can change words or make your pronunciation sound unnatural if ignored.

So:

• dokumentární has a long á
• dlouhý has a long ý

These are not just optional spelling marks; they are part of the word.

Yes. Film is a normal Czech noun, borrowed from the same international source as in English.

In Czech it behaves like a regular noun and has grammatical gender:

• film is masculine inanimate

That is why adjectives take masculine forms with it:

• dokumentární film
• dlouhý film

Yes, Czech word order is more flexible than English word order, but changing it often changes the emphasis.

The neutral order here is:

• Dokumentární film je dlouhý.

You could also say:

• Ten film je dlouhý. = That/The film is long.
• Dlouhý je ten dokumentární film. = marked/emphatic, something like It’s the documentary film that is long.

So although different orders are possible, the original sentence is the most natural neutral version.

They do two different jobs.

• dokumentární describes what kind of film it is: a documentary film
• dlouhý describes a quality of that film: it is long

So the structure is:

• Dokumentární film = noun phrase / subject
• je dlouhý = predicate

In other words:

• dokumentární belongs closely with film
• dlouhý comes after je and tells us something about the subject

In informal spoken Czech, you may hear dlouhej instead of dlouhý. That is a common colloquial form.

So:

• dlouhý = standard written Czech
• dlouhej = common informal spoken Czech

For learners, it is best to use dlouhý first, because it is the standard form and the one you should write in normal formal Czech.