Моя соседка жалуется, что в комнате душно.

Because соседка is a feminine noun, and the possessive my must agree with it in gender, number, and case.

• мой = masculine singular
• моя = feminine singular
• моё = neuter singular
• мои = plural

So:

• мой сосед = my male neighbor
• моя соседка = my female neighbor

Соседка means female neighbor.
Сосед means male neighbor or sometimes neighbor in a general masculine form.

Russian often distinguishes male and female people with different noun forms:

• сосед = male neighbor
• соседка = female neighbor

The ending -ка here helps form the feminine version.

Because the verb is жаловаться, which means to complain. It is a reflexive verb, so it includes -ся in its dictionary form.

Conjugation:

• я жалуюсь = I complain
• ты жалуешься = you complain
• он/она жалуется = he/she complains

So моя соседка жалуется means my neighbor complains or is complaining.

Without -ся, the verb would be a different word. Russian has many verbs where -ся is an essential part of the verb, not something optional.

Because it is the 3rd person singular form: she complains.

The subject is моя соседка, which is she, so the verb must match that:

• я жалуюсь
• ты жалуешься
• она жалуется

The ending -ется here is really:

• -ет = 3rd person singular verb ending
• -ся = reflexive particle

So жалуется = complains

Что here means that and introduces a subordinate clause.

So the sentence structure is:

• Моя соседка жалуется = My neighbor complains
• что в комнате душно = that it is stuffy in the room

Together:

• My neighbor complains that it is stuffy in the room.

In English, that is often optional, but in Russian что is very commonly used in this kind of sentence.

Because after в, Russian uses different cases depending on meaning.

Here, в комнате means in the room as a location, so Russian uses the prepositional case:

• комната = room (nominative)
• в комнате = in the room (prepositional)

Compare:

• в комнате = in the room, inside the room already
• в комнату = into the room, movement toward it

Since this sentence describes where it is stuffy, not movement, в комнате is correct.

Комнате is prepositional singular.

The noun is комната. Its singular forms include:

• комната = nominative
• комнаты = genitive
• комнате = dative / prepositional
• комнату = accusative
• комнатой = instrumental

After в when it means location, the prepositional is used:

• в комнате = in the room

So the ending -е signals the required case here.

Душно means stuffy, airless, or close in the sense of uncomfortable air.

It is not an adjective here in the usual sense. It is a predicative word (sometimes called a category-of-state word), used in impersonal sentences to describe an environment or condition.

Examples:

• Здесь холодно. = It is cold here.
• В комнате темно. = It is dark in the room.
• Мне скучно. = I am bored.
• В комнате душно. = It is stuffy in the room.

So Russian does not need a subject like it here.

Because Russian often omits the present-tense form of to be.

In English, you say:

• It is stuffy in the room.

In Russian, you simply say:

• В комнате душно.

There is no present-tense есть here. In modern Russian, to be in the present tense is usually left out in simple statements.

So the clause literally looks like:

• In the room stuffy

But the natural English translation is:

• It is stuffy in the room.

Жаловаться is imperfective.

That matters because the imperfective is used for:

• ongoing actions
• repeated actions
• general facts or habits

So Моя соседка жалуется can mean:

• My neighbor is complaining
• My neighbor complains

depending on context.

A perfective form would usually refer to a single completed act and would not normally be used the same way here.

Yes. Russian present tense often covers both meanings.

• Моя соседка жалуется can mean a general habit: My neighbor complains
• or something happening now: My neighbor is complaining

Context tells you which one is meant.

If this is just an isolated sentence, both are possible.

Yes, Russian word order is flexible, though the neutral order here is very natural:

• Моя соседка жалуется, что в комнате душно.

You could also hear:

• Моя соседка жалуется, что душно в комнате.

Both are understandable. The difference is mostly about emphasis or style, not core meaning.

In the original sentence, в комнате comes before душно, which sounds very normal when setting the scene first: in the room, it is stuffy.

Because что в комнате душно is a subordinate clause, and Russian normally separates subordinate clauses with a comma.

Main clause:

• Моя соседка жалуется

Subordinate clause:

• что в комнате душно

So the comma is required:

• Моя соседка жалуется, что в комнате душно.

This is standard Russian punctuation.