Learn Hardcore Russian: В начале фильма мне было скучно, а потом стало интересно. - At the beginning of the movie I was bored, but then it became interesting.

If you say я был скучный, it does not mean I was bored. It means I was a boring person (I myself was not interesting to others). That is a different idea.

So to say I was bored (during the film), the natural Russian is мне было скучно (на фильме / во время фильма), not я был скучный.

What grammatical structure is мне было скучно?

Мне было скучно is an impersonal sentence (a sentence without a normal grammatical subject) with a dative experiencer.

The structure is:

[Dative] + было / есть / будет + скучно / интересно / холодно / жарко …

Functionally, this means:

мне было скучно – I was bored
нам было весело – we had fun
детям было страшно – the children were scared

Grammatically, there is no noun in the nominative acting as subject; мне is in dative, and скучно is a special state word (see next question).

Why is the verb было in the neuter singular form?

In impersonal sentences, the past tense of быть usually appears in neuter singular (было), because there is no grammatical subject for it to agree with.

мне было скучно – literally to me it-was-boring, with было in neuter
здесь было холодно – it was cold here

The verb is not agreeing with мне (dative cannot be the grammatical subject), so Russian uses the default neuter singular form.

What exactly are скучно and интересно here: adjectives or adverbs?

Words like скучно, интересно, холодно, жарко, страшно in sentences such as мне скучно or мне интересно are usually called predicative adverbs or words of state.

Key points:

They do not change for gender or number: мне скучно, ей скучно, им скучно – always скучно.
They act as the predicate, describing a temporary state or feeling.
They are often used with a dative experiencer:
- мне интересно – I find it interesting
- ему скучно – he is bored

Historically they come from short-form adjectives, but for learners it is enough to remember:
мне скучно / интересно = a fixed way to say I am bored / I am interested.

What is the difference between мне было скучно, я скучал, and я был скучный?

They all look close to I was bored, but they are used differently.

мне было скучно
- Neutral, natural way to say I was bored in this context (during a film, lecture, etc.).
- Focus on your inner feeling.
я скучал
- Often means I was missing (someone/something) when used with по
  - dative:
    - я скучал по тебе – I missed you
- By itself it can mean I was bored, but in the context of a film it sounds less usual than мне было скучно.
- It sounds more like a process: I was in a state of boredom / I was pining.
я был скучный
- Means I was a boring person (others found me boring).
- Not used to mean I felt bored.

So, for the sentence about watching a film, мне было скучно is the correct and idiomatic choice.

What does стало интересно mean, and how is стало different from было?

Стало is the past tense (neuter singular) of the perfective verb стать – to become, to start to be.

стало интересно ≈ it became interesting / it started to be interesting
было интересно ≈ it was interesting

In the full structure мне стало интересно:

мне было скучно – I was bored (state in the past)
мне стало интересно – it became interesting for me (a change of state)

So стало emphasizes that something changed: boredom turned into interest. Using стало instead of было makes the contrast stronger.

Why is стало also in the neuter singular form?

For the same reason as было:

The sentence (мне) стало интересно is also impersonal.
There is no nominative subject for стало to agree with.
Russian uses the default neuter singular past form in such impersonal constructions.

So:

мне стало интересно – literally to me it-became-interesting, with стало neuter singular.

Do we need to repeat мне before стало интересно, or is it understood from context?

The fully explicit version is:

В начале фильма мне было скучно, а потом мне стало интересно.

In the original sentence, мне is not repeated:

… мне было скучно, а потом стало интересно.

This is very normal in Russian: when the same experiencer (here мне) continues in the second clause, it can be left out and understood from context.

So both are correct:

With repetition (more explicit): …, а потом мне стало интересно.
Without repetition (more natural and less heavy): …, а потом стало интересно.

Why is there a comma before а потом?

The sentence actually has two clauses:

В начале фильма мне было скучно
(мне) потом стало интересно

They are joined by the conjunction а. In Russian, when you join two independent clauses with а, you normally put a comma before а:

… было скучно, а потом стало интересно.

This is similar to English punctuation with but:
It was boring at first, but then it became interesting.

Why do we use а потом and not и потом here? Could we say и потом стало интересно?

Both а потом and и потом are grammatically possible, but they feel different.

а потом adds a sense of contrast or opposition.
Here it is like but then: first boring, but then interesting.
и потом is more neutral, more like and then, simply listing events.

In this sentence, we are contrasting скучно vs интересно, so а потом is the more natural choice.

You could say … мне было скучно, и потом стало интересно, but it weakens the feeling of contrast.

Could we change the word order, for example to Мне было скучно в начале фильма? Would that still be correct?

Yes, Russian allows quite flexible word order here. These are all possible:

В начале фильма мне было скучно.
Мне было скучно в начале фильма.
Мне в начале фильма было скучно.

Differences are mainly in emphasis:

Starting with в начале фильма highlights the time frame:
At the beginning of the film it was boring for me…
Starting with мне highlights the experiencer:
For me, at the beginning of the film, it was boring…

All of them are grammatically correct; the original order is very natural.

Where is the subject in this sentence? Why is there no it like in English it was boring?

Russian often uses impersonal sentences where English uses it as a dummy subject.

English: It was boring at the beginning of the film.
Russian: В начале фильма мне было скучно.
(literally: At the beginning of the film, to me was boring.)

There is no pronoun corresponding to English it; the sentence simply has:

a time phrase (в начале фильма),
a dative experiencer (мне),
a verb in neuter (было),
and a state word (скучно).

This no-explicit-subject pattern is very common in Russian.

Are there other natural ways to say the same idea in Russian?

Yes, a few close variants:

В начале фильма мне было скучно, а потом было интересно.
– same idea, but было in both parts; less focus on change.
Сначала мне было скучно, а потом стало интересно.
– сначала = at first; time is more general, not tied specifically to the beginning of the film.
В начале фильма было скучно, а потом стало интересно.
– here we drop мне, making it more general: it was boring (for people) at the beginning, then it became interesting.

The original sentence is very natural and clearly shows your personal reaction.

Where is the stress in the main words of the sentence?

Stress marks (the stressed vowel is marked with ´):

в нача́ле – na‑ча́‑ле (stress on ча)
фи́льма – фи́‑ль‑ма
мне – one syllable; just мне
бы́ло – бы́‑ло
ску́чно – ску́‑чно
пото́м – по‑то́м
ста́ло – ста́‑ло
интере́сно – ин‑те‑ре́с‑но (stress on ре́с)

So spoken rhythmically:
В нача́ле фи́льма мне бы́ло ску́чно, а пото́м ста́ло интере́сно.