Когда в комнате душно, мне тяжело дышать.

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

So:

• в комнате душно literally looks like in the room stuffy
• but it means it is stuffy in the room

This is very common in Russian:

• Мне холодно = I am cold / literally To me [it is] cold
• Дома тихо = It is quiet at home

If you wanted past or future, Russian would use forms of быть:

• В комнате было душно = The room was stuffy
• В комнате будет душно = The room will be stuffy

Душно means something like:

• stuffy
• close
• airless
• oppressively warm / lacking fresh air

It is not just hot. A room can be жарко if it is hot, but душно specifically suggests that the air feels heavy and uncomfortable, often because there is not enough ventilation.

So Когда в комнате душно means more than just when the room is warm. It suggests when the room feels stuffy and hard to breathe in.

Because the preposition в here means in, and with location it requires the prepositional case.

• комната = nominative
• в комнате = prepositional, meaning in the room

This is a standard pattern:

• в доме = in the house
• в школе = at school / in the school
• в машине = in the car

So в комнате simply means in the room.

Because Russian often expresses physical or emotional states with the dative case, especially in impersonal constructions.

So:

• мне тяжело дышать literally means to me, it is hard to breathe
• natural English: it is hard for me to breathe or I have trouble breathing

This pattern is extremely common:

• мне холодно = I am cold
• мне скучно = I am bored
• мне трудно понять = It is hard for me to understand

Using я here would not fit this construction.

It is an impersonal construction.

Breakdown:

• мне = to me / for me
• тяжело = hard / difficult / тяжело
• дышать = to breathe

So the structure is:

[dative person] + [predicative word] + [infinitive]

Meaning:

It is hard for someone to do something

Examples:

• Мне трудно говорить = It is hard for me to speak
• Ему легко учиться = It is easy for him to study
• Нам приятно работать вместе = It is pleasant for us to work together

So мне тяжело дышать = it is hard for me to breathe.

Yes, тяжёлый / тяжело often relates to weight or heaviness, but it also has a broader meaning: difficult, hard, or oppressive.

In this sentence, тяжело дышать means:

• it is hard to breathe
• breathing feels difficult/oppressive

This is a very natural expression in Russian, especially for physical discomfort.

Compare:

• трудно дышать = hard to breathe
• тяжело дышать = hard to breathe, often with a stronger physical or oppressive feeling

Here тяжело fits very well because душно creates exactly that kind of heavy, oppressive sensation.

Yes, you could. Both are natural, but they are not always identical in tone.

• трудно дышать = difficult to breathe
• тяжело дышать = hard to breathe, often with more emphasis on physical strain or discomfort

In this sentence, тяжело sounds especially appropriate because the room is stuffy, so the difficulty feels physical and oppressive.

So:

• Когда в комнате душно, мне трудно дышать = correct
• Когда в комнате душно, мне тяжело дышать = also correct, and often a bit more vivid

Here когда can be understood as when in a general sense, very close to whenever.

The sentence is not about one specific moment only. It expresses a general truth or repeated experience:

• Когда в комнате душно, мне тяжело дышать.
• When/Whenever the room is stuffy, it is hard for me to breathe.

Russian often uses когда this way for general situations.

If you wanted a more conditional flavor, you might also hear если in some contexts, but когда is very natural here because it presents the situation as something that happens and leads to a result.

Yes, дышать is imperfective.

That makes sense because the sentence describes a general process/state, not a completed action. Breathing is an ongoing activity, so Russian uses the imperfective infinitive.

You would not normally use a perfective verb here, because there is no idea of finishing or successfully completing the act of breathing. The point is simply that the process itself is difficult.

So дышать is exactly the right form.

Yes. Russian word order is flexible, although different orders can sound more natural in different contexts.

The neutral version is:

• Когда в комнате душно, мне тяжело дышать.

Other possible orders include:

• Мне тяжело дышать, когда в комнате душно.
• Когда душно в комнате, мне тяжело дышать.

These all mean basically the same thing, but the focus can shift slightly.

• Starting with Когда в комнате душно sets up the situation first.
• Starting with Мне тяжело дышать emphasizes your experience first.

For learners, the original order is a very good standard model.

In this sentence, душно is not an ordinary adjective modifying a noun. It is a predicative word used in an impersonal sentence.

That means it functions like a state:

• Душно = It is stuffy
• Холодно = It is cold
• Темно = It is dark

These words often describe environment, weather, or physical/emotional states without a normal subject.

So в комнате душно is not the room is stuffy in the same grammatical way English does it; in Russian it is more like in the room, it is stuffy.

Yes. Depending on context, мне тяжело дышать can be translated as:

• It is hard for me to breathe
• I have difficulty breathing
• I find it difficult to breathe

All of these capture the idea well.

The Russian structure is less about a personal action with I as grammatical subject, and more about a state affecting the person. That is why the dative мне is used.

It is very neutral and natural. You could say it in everyday speech, and it would also be fine in writing.

It does not sound especially formal or especially slangy. It is a normal standard Russian sentence.

Very similar natural alternatives would be:

• Когда в комнате душно, мне трудно дышать.
• Если в комнате душно, мне тяжело дышать.

But the original sentence is completely standard and idiomatic.