Вчерашний фильм был документальным, но очень интересным.

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Breakdown of Вчерашний фильм был документальным, но очень интересным.

• интересный: interesting
• фильм: the movie
• но: but

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.