Learn Hardcore Russian: Когда я смотрю фильм, я уменьшаю громкость, чтобы не мешать соседям. - When I watch a movie, I lower the volume so as not to disturb the neighbors.

Когда я смотрю фильм is a subordinate (time) clause introduced by когда (when).
я уменьшаю громкость, чтобы не мешать соседям is the main clause.

So you put a comma between them:
Когда я смотрю фильм, я уменьшаю громкость…

Can I leave out the second я and just say Когда я смотрю фильм, уменьшаю громкость…?

That would sound incomplete or wrong.

In Russian, you normally repeat the subject pronoun я when two separate clauses share the same subject, especially if there is a comma between them:

Когда я смотрю фильм, я уменьшаю громкость… – natural.

Dropping я after the comma is something you can do only in very limited, special stylistic contexts. In standard speech here, you keep я.

Why is the verb смотрю (imperfective) used instead of посмотрю?

Смотреть (imperfective) is used for:

repeated/habitual actions,
ongoing processes,
general statements.

The sentence describes what you usually do when you watch a film, so the imperfective present смотрю is correct:

Когда я смотрю фильм, я уменьшаю громкость… – whenever/when I watch a film, I turn the volume down.

Посмотрю (perfective) would mean I will watch (once, to completion) and doesn’t fit this “every time / whenever” meaning.

Can I say Когда я смотрю фильмы instead of Когда я смотрю фильм?

Yes, you can, and it slightly changes the nuance.

Когда я смотрю фильм… – focuses more on a single film at a time: when I’m watching a (any) film…
Когда я смотрю фильмы… – emphasizes films in general, plural: when I watch movies (in general)…

Both are grammatically correct; the original is just a very typical way to talk about this habitual situation.

Why doesn’t фильм change its form in the sentence? It’s still фильм.

Фильм is a masculine noun with the same form in nominative and accusative when it is singular and inanimate:

Nominative: фильм – the film is interesting (Фильм интересный)
Accusative: фильм – I’m watching a film (Я смотрю фильм)

So in я смотрю фильм, фильм is in the accusative case, but it just looks the same as the nominative.

What’s the difference between фильм and кино?

Both can relate to movies, but they’re used differently.

Фильм = a specific movie/film (countable):
- Этот фильм интересный. – This film is interesting.
Кино can mean:
- “movies” as an art form or entertainment: Я люблю кино. – I like movies / cinema.
- “the cinema” (informal): Пойдём в кино. – Let’s go to the movies.

In this sentence, you’re talking about one movie you’re watching, so фильм is the natural choice.

What exactly does уменьшаю громкость mean, and why this verb?

Уменьшаю громкость literally means “I reduce the volume” / “I lower the volume.”

уменьшать = to reduce, to make smaller (imperfective)
громкость = volume (of sound)

So я уменьшаю громкость = I turn the volume down.

You could also say more colloquially:

я делаю тише (literally “I make it quieter”)
я убавляю громкость

But уменьшаю громкость is clear and neutral.

Which case is громкость in, and why?

Громкость is in the accusative case, because it is the direct object of the verb:

Verb: уменьшаю (I reduce)
Object: громкость (what? volume)

Declension: громкость is a feminine noun ending in -ь. For such nouns, the nominative singular and accusative singular look the same: громкость → громкость.

Why is there a comma before чтобы in я уменьшаю громкость, чтобы не мешать соседям?

Because чтобы не мешать соседям is a separate subordinate clause expressing purpose.

Structure:

Main clause: я уменьшаю громкость
Subordinate purpose clause: (для того,) чтобы не мешать соседям – in order not to disturb the neighbors

Russian normally separates the main clause and a purpose clause with чтобы by a comma.

How does чтобы не мешать work grammatically here?

Чтобы + infinitive often expresses purpose: in order to do something.

чтобы мешать – in order to disturb
чтобы не мешать – in order not to disturb

So уменьшаю громкость, чтобы не мешать соседям = I lower the volume so as not to disturb the neighbors.

Note that мешать is in the infinitive, not conjugated, because the subject is understood to be the same я as in the main clause.

What does мешать mean exactly, and which case does it use?

Мешать has several meanings, but here it means “to disturb / to bother / to interfere with”.

With this meaning, мешать takes a dative object:

мешать кому? – to disturb whom?

Examples:

Не мешай мне. – Don’t disturb me. (мне – dative)
Он мешает соседям. – He bothers the neighbors. (соседям – dative)

So in the sentence, соседям is dative plural after не мешать.

Why is it соседям and not something like соседи?

Соседям is the dative plural form of сосед (neighbor).

Declension of сосед (masculine):

Nominative plural: соседи – neighbors (as the subject)
Dative plural: соседям – to/for neighbors

Because мешать uses the dative (мешать кому?), you must say:

не мешать соседям – not disturb the neighbors.

Could I say моим соседям instead of just соседям?

Yes: чтобы не мешать моим соседям is perfectly correct and a bit more explicit: so as not to disturb my neighbors.

However, Russian often omits possessive pronouns like мой, твой, наш, ваш when it’s obvious whose something is (family, neighbors, house, etc.).

So соседям alone is natural and fully understood as “my neighbors” in this context.

Can I change the word order, for example: Когда я фильм смотрю, я громкость уменьшаю?

That sentence is grammatically possible, but the word order sounds marked or stylistic.

The neutral, most natural order in modern Russian is:

Когда я смотрю фильм, я уменьшаю громкость…

Moving фильм before смотрю and громкость before уменьшаю can sound poetic, emphatic, or old-fashioned in everyday speech. For standard usage, it’s better to keep the given word order.