Film nebyl krátký, naopak byl dlouhý.

Czech does not use articles like a/an and the.

So film can mean:

• a film
• the film

You understand which one is meant from context. In this sentence, English would usually say the film, but Czech simply says film.

Nebyl means was not.

It is made from:

• byl = was
• ne- = the negative prefix

So:

• byl = he/it was
• nebyl = he/it was not

In Czech, negation is often formed by adding ne- directly to the verb.

Because the verb in the past tense agrees with the subject in gender and number.

The subject is film, which is:

• masculine inanimate
• singular

So the correct past-tense form is:

• byl = masculine singular
• nebyl = masculine singular negative

Compare:

• film byl = the film was
• kniha byla = the book was
• město bylo = the city was

These adjectives agree with film in gender, number, and case.

Since film is:

• masculine inanimate
• singular
• here functioning as the subject

the adjectives also take the masculine singular nominative form:

• krátký = short
• dlouhý = long

So:

• film byl krátký
• film byl dlouhý

If the noun changed, the adjective would change too.

After the verb být (to be), Czech normally uses the nominative for predicate adjectives.

So:

• Film byl dlouhý.
• Film nebyl krátký.

This is different from some other structures in Czech where another case might appear, but with ordinary adjective descriptions after být, nominative is the normal choice.

Naopak means something like:

• on the contrary
• quite the opposite
• rather

In this sentence, it strengthens the contrast:

• Film nebyl krátký = The film was not short
• naopak byl dlouhý = on the contrary, it was long

So it does more than just connect two ideas; it shows that the second idea is the opposite of what might have been expected.

Yes, you could say:

Film nebyl krátký, ale byl dlouhý.

That means:

• The film was not short, but long.

The difference is tone:

• ale = but → simple contrast
• naopak = on the contrary → stronger, more emphatic contrast

So naopak sounds a bit more pointed.

Because Czech often leaves out words that are already clear from context.

In:

• Film nebyl krátký, naopak byl dlouhý.

the subject of the second clause is still film, so Czech does not need to repeat it.

You could repeat it for emphasis:

• Film nebyl krátký, naopak film byl dlouhý.

But that sounds less natural in a neutral sentence.

No, Czech word order is relatively flexible, but different orders can sound more or less natural depending on emphasis.

The given sentence:

• Film nebyl krátký, naopak byl dlouhý.

is a very natural neutral version.

Other possible versions include:

• Film krátký nebyl, naopak byl dlouhý.
• Krátký film nebyl, naopak byl dlouhý.
  This changes the emphasis.

A learner should usually treat the original version as the safest default.

The comma separates the two parts of the sentence.

Here, naopak introduces a contrasting second clause:

• Film nebyl krátký, naopak byl dlouhý.

In English, you might also use a semicolon or dash in a similar sentence:

• The film wasn’t short; on the contrary, it was long.

In Czech, the comma is normal here.

A few useful pronunciation points:

• á is a long vowel
• ou is a diphthong, roughly like oh-oo said together
• ý is a long ee-like sound, though not exactly the same as English ee
• the stress in Czech is almost always on the first syllable

So roughly:

• krátký ≈ KRAAT-kee
• dlouhý ≈ DLOH-hee / DLOU-hee

This is only an approximation, but it helps with the rhythm.

Yes. Czech film can correspond to English film or movie, depending on context and translation style.

So natural English translations include:

• The film wasn’t short; on the contrary, it was long.
• The movie wasn’t short; it was long.

The Czech grammar stays exactly the same.