Если в комнате жарко, открой окно.

In Russian, a comma is normally used to separate the if-clause from the main clause.

• Если в комнате жарко, открой окно.
• literally: If in the room it is hot, open the window.

So the comma works much like English If it’s hot in the room, open the window.

If the order changes, the comma is still usually needed:

• Открой окно, если в комнате жарко.

В комнате means in the room.

Here, в means in, and because it shows location, комната goes into the prepositional case:

• комната → nominative
• в комнате → prepositional

This is a very common pattern:

• в доме = in the house
• в школе = in the school
• в комнате = in the room

Жарко here is not an adjective describing a noun. It is a predicative adverb or category of state word, used to describe a condition or environment.

So:

• В комнате жарко = It’s hot in the room

There is no noun being described directly. You are not saying the room is hot with an adjective matching room in gender and number. Instead, you are describing the general state of the room.

Compare:

• жаркая комната = a hot room
• в комнате жарко = it’s hot in the room

This kind of word is very common in Russian:

• холодно = cold
• темно = dark
• тихо = quiet
• душно = stuffy

Russian often does not use a dummy subject like English it.

English says:

• It is hot in the room

Russian simply says:

• В комнате жарко

There is no need for a separate subject. This is normal in Russian with weather, conditions, and general states.

Other examples:

• Холодно. = It’s cold.
• Темно. = It’s dark.
• Поздно. = It’s late.

Открой is the imperative of the perfective verb открыть.

Russian often uses:

• perfective imperative for a single complete action
• imperfective imperative for a repeated process, general instruction, or emphasis on the action itself

So:

• открой окно = open the window / open the window now
• открывай окно = start opening the window / open the window regularly / go ahead and open it

In this sentence, the speaker means perform one complete action if the room is hot, so открой is the natural choice.

It comes from the verb открыть = to open.

The ты command form is:

• открыть → открой

This means open! when speaking to one person informally.

Related forms:

• открой(те) = open!
• откройте = open! (to more than one person, or politely to one person)

So if you are speaking politely, you would say:

• Если в комнате жарко, откройте окно.

Because окно is the direct object of открой, so it is in the accusative case.

However, окно is a neuter inanimate noun, and for many neuter inanimate nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: окно
• accusative: окно

That is why the form does not change.

Compare with another neuter noun:

• письмо = letter
• читать письмо = to read a letter

Same pattern.

In Russian, the subject is often omitted when it is clear from the verb form.

Открой is a command to you singular informal, so ты is understood automatically.

• (Ты) открой окно. = Open the window.

Russian usually leaves ты out unless it is being emphasized.

If you include it, it can sound more emphatic:

• Ты открой окно. = You open the window.

Yes. Russian word order is flexible, and that version is grammatical.

Compare:

• Если в комнате жарко, открой окно.
• Если жарко в комнате, открой окно.

Both mean the same thing. The first version is a bit more neutral and natural in many contexts. The difference is mostly about emphasis and flow, not basic meaning.

Russian often moves parts of the sentence around for style or emphasis.

Here, если means if and introduces a condition:

• Если в комнате жарко, открой окно.

Yes, если is the standard word for if in conditional sentences.

Very common patterns:

• Если будет дождь, останемся дома. = If it rains / if there is rain, we’ll stay home.
• Если хочешь, помогу. = If you want, I’ll help.

It is a general real condition: if the room is hot, open the window.

Russian often uses the present tense in the if-clause even when the meaning is practical or future-oriented in English. The idea is:

• whenever that condition is true, do this

So it is less about strict tense and more about a real condition plus an instruction.

Жарко is the normal word for saying that the air or the environment feels hot.

• В комнате жарко. = It’s hot in the room.

Горячо usually means hot to the touch or hot in a more direct physical sense:

• Чай горячий. = The tea is hot.
• Мне горячо. can mean I feel too hot, but for a room or environment жарко is more natural.

So for temperature in a room, жарко is the expected choice.

Word-for-word, yes, something close to that is what the Russian structure looks like. But natural English needs extra words:

• Если = if
• в комнате = in the room
• жарко = hot / it is hot
• открой = open
• окно = window / the window

Natural English adds it is and often the:

• If it’s hot in the room, open the window.

This is a good reminder that Russian often leaves out words that English requires, especially it and articles like a/the.

Russian has no articles, so it does not use words like a, an, or the.

That means:

• в комнате can mean in the room or in a room
• окно can mean window or the window

The exact meaning depends on context. In this sentence, English would usually say the room and the window, but Russian does not mark that explicitly.

Yes.

• открой = command to one person informally
• откройте = command to more than one person, or polite singular

So:

• Если в комнате жарко, открой окно. = informal, to one person
• Если в комнате жарко, откройте окно. = polite, or addressed to several people

This is a very important distinction in Russian.

Открыть is perfective. Its usual imperfective partner is открывать.

• открывать = to open, to be opening, to open repeatedly
• открыть = to open completely, to finish opening

Examples:

• Я открываю окно. = I am opening the window / I open the window.
• Я открыл окно. = I opened the window.

In your sentence, the command uses the perfective because the speaker wants one completed action:

• открой окно

Yes, but it means something slightly different.

• Если в комнате жарко = if the room is hot / if it’s hot in the room
• Если тебе жарко = if you feel hot

The first focuses on the room’s conditions. The second focuses on how the person feels.

Sometimes both work in context, but they are not exactly the same.