Не стучи так громко: ребёнок уже уснул в соседней комнате.

Russian often leaves out subject pronouns when they are clear from the verb form.

Here, стучи is the singular informal imperative, so it already means knock / don’t knock when speaking to one person. Because of that, ты is unnecessary.

If you wanted to include it for emphasis, you could say Ты не стучи так громко, but that is less neutral.

Стучи is the imperative singular form of стучать.

• infinitive: стучать = to knock, to bang, to tap
• imperative singular: стучи = knock
• with не: не стучи = don’t knock

Many Russian verbs form the singular imperative with -и:

• говорить → говори
• смотреть → смотри
• стучать → стучи

So не стучи is the normal way to tell one person don’t knock / don’t bang.

In this sentence, стучать means making repeated knocking, banging, or tapping sounds.

Depending on context, it could mean:

• knocking on a door
• tapping on a wall or table
• banging noisily

So Не стучи так громко is more generally Don’t make that knocking/banging noise so loudly, not only a formal don’t knock at the door.

Because громко is an adverb, and it describes how the action is done.

• громкий = loud, noisy (adjective)
• громко = loudly (adverb)

Here it modifies the verb стучи:

• стучи громко = knock loudly

An adjective like громкий would need a noun:

• громкий стук = a loud knock

Here так means so.

• громко = loudly
• так громко = so loudly

It adds emphasis, like English Don’t knock so loudly.

In other contexts, так can also mean like this, this way, or so, depending on the sentence.

The colon introduces the reason or explanation for the command.

So the structure is basically:

• Не стучи так громко:
  don’t knock so loudly
• ребёнок уже уснул в соседней комнате.
  the child has already fallen asleep in the next room

It works like English Don’t knock so loudly: the child has already fallen asleep in the next room.

Russian could also use a conjunction, for example:

• Не стучи так громко, потому что ребёнок уже уснул...
• Не стучи так громко — ребёнок уже уснул...

But the colon is a natural written way to show here is the reason.

The correct pronunciation is with ё: ребёнок.

In everyday Russian writing, ё is often replaced by е, so you may see ребенок, but it is still pronounced ребёнок.

For learners, it is very helpful to pay attention to ё, because it affects pronunciation and sometimes meaning. Here:

• ребёнок = child

The stress is on the ё.

Уснул is the past tense masculine singular form of уснуть, which means to fall asleep.

So:

• уснуть = to fall asleep
• уснул = fell asleep / has fallen asleep

It is masculine because it agrees with ребёнок, which is grammatically masculine.

Compare:

• ребёнок уснул = the child fell asleep
• девочка уснула = the girl fell asleep
• дети уснули = the children fell asleep

Because уснул focuses on the completed action of falling asleep.

• уснул = fell asleep / has fallen asleep
• спит = is sleeping

This sentence is explaining why the person should be quiet: the child has already gone to sleep. That completed result is important.

If you said ребёнок спит, that would mean the child is sleeping, which is also possible, but slightly different in focus.

• ребёнок уже уснул = the child has already fallen asleep
• ребёнок уже спит = the child is already asleep / sleeping

Уже means already.

It shows that the action has happened by this point:

• ребёнок уснул = the child fell asleep
• ребёнок уже уснул = the child has already fallen asleep

It makes the warning more natural: be quiet, because this has already happened.

This is the prepositional case after в when it means in a place.

• комната = room
• в комнате = in the room

Because соседней describes комнате, it must also be in the same case:

• nominative: соседняя комната = neighboring room
• prepositional: в соседней комнате = in the neighboring room

So:

• в = in
• соседней = neighboring, in prepositional feminine singular
• комнате = room, in prepositional singular

Because the sentence means in the neighboring room / in the next room — one room.

• в соседней комнате = in the neighboring room
• в соседних комнатах = in the neighboring rooms

English sometimes uses room or rooms differently in similar situations, but here Russian clearly refers to one nearby room.

Yes. Ребёнок is grammatically masculine, regardless of the child’s actual sex.

So you say:

• ребёнок уснул even if you mean a girl, if you are using the noun ребёнок

But if you use a specifically feminine noun, then the verb changes:

• девочка уснула = the girl fell asleep

This is a good example of the difference between grammatical gender and real-world gender.