Вчерашний фильм был документальным, но очень интересным.

Yesterday’s movie was a documentary but very interesting.

Breakdown of Вчерашний фильм был документальным, но очень интересным.

• быть: to be
• интересный: interesting
• фильм: the movie
• но: but
• очень: very
• вчерашний: yesterday's
• документальный: documentary

Questions & Answers about Вчерашний фильм был документальным, но очень интересным.

What exactly does вчерашний mean, and how is it different from вчера?

Вчера is an adverb meaning yesterday (it tells you when something happened):

• Я смотрел фильм вчера. – I watched the film yesterday.

Вчерашний is an adjective meaning yesterday’s / from yesterday (it describes a noun):

• вчерашний фильм – yesterday’s film / the film from yesterday
• вчерашняя газета – yesterday’s newspaper
• вчерашний ужин – yesterday’s dinner (leftovers)

So in Вчерашний фильм был документальным, the idea is literally “The film from yesterday was a documentary”.

Why does Russian say вчерашний фильм, not something like “фильм вчера”?

Both structures exist, but the nuance is different:

• Вчерашний фильм был документальным…
  Literally “Yesterday’s film was documentary…”
  You are identifying which film (the one from yesterday).

• Фильм вчера был документальным…
  Literally “The film yesterday was documentary…”
  This sounds a bit unusual without more context; more natural would be:
  Фильм, который мы смотрели вчера, был документальным. – The film that we watched yesterday was a documentary.

Using вчерашний is a compact way to say “the film from yesterday” and is very natural in Russian.

Why do we need был in the sentence? In English we can sometimes drop “was”.

In Russian:

• In the present tense, the verb быть (to be) is usually omitted:

  • Фильм интересный. – The film is interesting. (no verb in Russian)

• In the past or future, you must use a form of быть:

  • Фильм был интересным. – The film was interesting.
  • Фильм будет интересным. – The film will be interesting.

So in the past tense, a sentence like Фильм документальный sounds incomplete; standard Russian needs был:

• Фильм был документальным. – The film was a documentary.

Why is it документальным and not документальный?

Документальным is in the instrumental case (masculine singular), because it is a predicate adjective after был:

• Фильм был документальным.
  Subject: фильм (nominative)
  Predicate: документальным (instrumental, agrees in gender and number with фильм)

Rule (simplified):
After быть (and some similar verbs), when you say what something is / was / will be (profession, type, role, characteristic), Russian very often uses the instrumental case:

• Он был учителем. – He was a teacher.
• Она будет врачом. – She will be a doctor.
• Фильм был документальным. – The film was documentary.

Документальный (nominative) would normally be used before the noun, as an ordinary descriptive adjective:

• документальный фильм – a documentary film

Why is интересным also in the instrumental case? Could it be интересный?

Интересным is also in the instrumental to match the pattern:

• (Фильм) был документальным, но (был) очень интересным.

Here был is understood before очень интересным but not repeated. Both документальным and интересным are predicates tied to the same verb был, so they share the same case: instrumental.

You can hear colloquial speech like:

• Фильм был документальный, но очень интересный.

Here both adjectives are in the nominative; this feels more like “The film was (kind of) documentary, but (still) very interesting.” It is widely used in conversation, but the instrumental is more standard and neutral, especially in writing:

• Фильм был документальным, но очень интересным.

What does но mean here, and why is there a comma before it?

Но means “but”.

The comma is there because но connects two parts of the predicate:

• (Фильм) был документальным, но (был) очень интересным.

In Russian, a comma is normally placed before coordinating conjunctions like но when they link two clauses or two separate parts of a predicate that each could be expanded to a clause:

• Он устал, но продолжал работать. – He was tired, but continued to work.
• Фильм был скучным, но полезным. – The film was boring but useful.

Compare:

• документальный, но интересный фильм – no comma, because these are two adjectives directly before the same noun.

Could we say Документальный, но очень интересный фильм был вчера instead? Is that correct?

Yes, that is grammatically correct, but the focus changes.

Original:

• Вчерашний фильм был документальным, но очень интересным.
  Focus: the film from yesterday; then you describe it.

Variant:

• Документальный, но очень интересный фильм был вчера.
  Literally: “A documentary but very interesting film was yesterday.”
  This sounds like you are first describing what kind of film it was, and only at the end add “(it) was yesterday.”

Both are correct, but the original sentence is more natural if the main thing in your mind is “yesterday’s film” as a specific known film.

How do the adjective endings relate to gender and number in this sentence?

The noun фильм is masculine singular.

Therefore, all adjectives referring to it agree in gender and number:

• вчерашний – masculine singular, nominative (-ий ending)
• документальным – masculine singular, instrumental (-ым ending)
• интересным – masculine singular, instrumental (-ым ending)

If we change the noun, the forms change:

• Feminine:
  Вчерашняя передача была документальной, но очень интересной.
  (передача – feminine)

• Plural:
  Вчерашние фильмы были документальными, но очень интересными.

What is the difference between документальный фильм and фильм был документальным?

• документальный фильм – an adjective + noun combination.
  You are naming a type of film: a documentary film.

• фильм был документальным – a sentence telling you what the film was.
  You usually say this about a specific film already known in the conversation:
  Тот фильм был документальным. – That film was a documentary.

Often you could choose either, depending on style:

• Мы смотрели документальный фильм. – We watched a documentary.
• Фильм, который мы смотрели, был документальным. – The film we watched was a documentary.

The second sounds a bit more like a full statement or explanation.

Is the word вчерашний used only literally (“of yesterday”), or does it have figurative meanings too?

Besides the literal meaning “from yesterday”, вчерашний can be used figuratively to mean something like “old-fashioned, outdated, behind the times”:

• Он человек вчерашний. – He’s old-fashioned / stuck in the past.
• Это уже вчерашняя новость. – That’s yesterday’s news (i.e. old news).

In your sentence Вчерашний фильм был документальным…, it is purely literal: the film you watched yesterday.

Can очень go in a different position, or must it be очень интересным?

The normal, natural position for очень is directly before the adjective or adverb it modifies:

• очень интересным – very interesting
• очень скучный фильм – a very boring film

You might occasionally see интересным, очень in special expressive contexts (for emphasis, in poetry, or stylized speech), but for everyday Russian you should almost always put очень before the adjective:

• Фильм был документальным, но очень интересным. ✅ (normal, idiomatic)