Фильм был невероятно интересным.

Because after быть in the past tense, Russian commonly uses the instrumental case for the predicate adjective.

• Dictionary form: интересный
• Instrumental masculine singular: интересным

So:

• Фильм = subject
• был = was
• невероятно интересным = incredibly interesting

For a learner, был + instrumental is the safest pattern to use in sentences like this.

Был is the past tense masculine singular form of быть (to be).

It matches фильм, which is:

• masculine
• singular

So Russian chooses:

• был for masculine singular
• была for feminine singular
• было for neuter singular
• были for plural

Examples:

• Книга была интересной. = The book was interesting.
• Письмо было интересным. = The letter was interesting.
• Фильмы были интересными. = The films were interesting.

The dictionary form is интересный, meaning interesting.

In this sentence it changes to интересным because it is:

• masculine singular
• instrumental case

This adjective agrees with фильм in gender and number.

A useful pattern to remember is:

• интересный фильм = an interesting film
• фильм был интересным = the film was interesting

So the adjective has one form when it directly describes the noun, and another form when it is used after был.

Невероятно is an adverb meaning incredibly, unbelievably, or extremely depending on context.

In this sentence it modifies the adjective интересным:

• интересным = interesting
• невероятно интересным = incredibly interesting

This is similar to English:

• very interesting
• really interesting
• incredibly interesting

Other examples:

• невероятно красивый = incredibly beautiful
• невероятно трудно = incredibly difficult

Because in Russian, adverbs that modify adjectives usually come before the adjective, just as in English.

So:

• невероятно интересным = incredibly interesting

This is the normal word order.

You could change the word order in some contexts for emphasis, but the neutral version is the one you see here.

Russian has no articles, so there is no direct equivalent of the or a/an.

So Фильм can mean:

• the film
• a film

Context tells you which one is meant.

If Russian wants to be more specific, it can use words like:

• этот фильм = this film
• какой-то фильм = some film

But in a simple sentence like this, no article is needed.

Not in the past tense.

In the present tense, Russian usually leaves out есть in sentences like this:

• Фильм интересный. = The film is interesting.

But in the past tense, you must use the past form of быть:

• Фильм был интересным. = The film was interesting.

So:

• present: Фильм интересный
• past: Фильм был интересным

That is a very important pattern in Russian.

Yes, Russian word order is more flexible than English, but the original order is the most neutral.

Neutral:

• Фильм был невероятно интересным.

Other possible orders might be used for emphasis:

• Невероятно интересным был фильм.
• Интересным фильм был невероятно.
  This one is much less natural in ordinary speech and sounds highly marked.

For everyday use, stick with:

• Фильм был невероятно интересным.

Yes. A common alternative is to use the short-form adjective:

• Фильм был невероятно интересен.

This also means The film was incredibly interesting.

The difference is mostly stylistic:

• был невероятно интересным = neutral, very common
• был невероятно интересен = a bit more literary or formal

For most learners, the full adjective with instrumental, интересным, is easier and safer to use first.

The stress is:

• Фи́льм был невероя́тно интере́сным.

Stress matters in Russian pronunciation, so it helps to learn words together with stress.

A rough pronunciation guide:

• Фильм ≈ feelm
• был ≈ byl
• невероятно ≈ nyee-vye-ree-YAT-na
• интересным ≈ een-tye-RES-nyim

The exact sounds are not the same as English, but the stress pattern above is the important starting point.

Here is the breakdown:

• Фильм — subject, nominative singular
• был — past tense verb, masculine singular
• невероятно — adverb
• интересным — predicate adjective, instrumental singular

So the sentence structure is:

• subject + was + adverb + adjective

That makes it a very useful model for building similar sentences, for example:

• Урок был очень трудным. = The lesson was very difficult.
• Концерт был довольно скучным. = The concert was rather boring.
• Разговор был необычайно полезным. = The conversation was unusually useful.