Не свисти в комнате, ребёнок уже спит.

Свисти is the singular informal imperative of свистеть = to whistle.

So:

• свистеть = to whistle
• свисти = whistle!
• не свисти = don’t whistle

This is the form you use when speaking to one person in an informal way, for example a child, friend, or family member.

Because Russian usually uses the imperfective imperative for a general negative command or prohibition.

Here:

• свистеть = imperfective, to whistle
• не свисти = don’t whistle

This sounds natural for don’t do that in a general sense.

A perfective version such as не свистни would sound more like don’t suddenly give a whistle or don’t whistle once, which is not the neutral everyday choice here.

Russian often omits subject pronouns when they are obvious from the verb form.

In English, you usually need you:

• Don’t whistle

In Russian, the imperative already shows that someone is being addressed, so ты is unnecessary:

• Не свисти

If you added ты, it would usually sound more emphatic:

• Ты не свисти = You don’t whistle / Don’t you whistle

So the version without ты is the normal neutral one.

Because в here means in, and комнате is the prepositional case after в when talking about location.

Base form:

• комната = room

After в for location:

• в комнате = in the room

This is a very common pattern:

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

Because Russian distinguishes location from direction.

• в комнате = in the room → location, where something happens
• в комнату = into the room → direction, movement toward a place

In this sentence, the person is being told not to whistle while in the room, so location is needed:

• Не свисти в комнате = Don’t whistle in the room

If you said в комнату, it would suggest movement into the room, which does not fit here.

Уже means already.

In this sentence it adds the idea that the child has already fallen asleep or is already sleeping now, so whistling would disturb them.

It often gives a sense of:

• something has happened by now
• the situation is already true
• this is the reason you should adjust your behavior

Examples:

• Я уже дома = I’m already home
• Он уже ест = He’s already eating

Because Russian does not have a separate tense form like English is sleeping.

The verb спит is the 3rd person singular present of спать:

• спать = to sleep
• спит = sleeps / is sleeping

Which English translation fits depends on context.

Here, because the situation is happening right now, спит is naturally understood as:

• the child is sleeping

So Russian present tense often covers both:

• habitual meaning: sleeps
• ongoing meaning: is sleeping

Because the sentence has two clauses:

• Не свисти в комнате
• ребёнок уже спит

Russian often connects clauses like this with a comma, even without a conjunction such as because.

The relationship here is basically causal:

• Don’t whistle in the room, the child is already sleeping.

In English, you might also say:

• Don’t whistle in the room, because the child is already sleeping.

Russian does not need потому что here, though it could be added in a different sentence:

• Не свисти в комнате, потому что ребёнок уже спит.

Because the standard spelling of this word is ребёнок.

Important points:

• ё shows both the vowel sound and the stress
• ребёнок is stressed on ё
• in many everyday texts, Russians often write е instead of ё, so you may see ребенок

But for learners, it is best to remember the correct form as:

• ребёнок

This helps both with pronunciation and with recognizing the word.

A helpful stressed version is:

Не свисти́ в ко́мнате, ребёнок уже́ спит.

Key stress:

• свисти́
• ко́мнате
• ребёнок
• уже́
• спит

A few pronunciation notes:

• не is usually unstressed
• ё is always stressed when written
• unstressed vowels are reduced in natural speech, especially о

So even though комнате is spelled with о, the first vowel is clearer because it is stressed: ко́м-.

Use the plural / polite imperative:

• Не свистите в комнате, ребёнок уже спит.

So:

• не свисти = don’t whistle, to one person informally
• не свистите = don’t whistle, to several people or to one person politely

This is one of the most useful Russian imperative patterns to learn:

• singular informal: -и
• plural / polite: -ите