Learn Hardcore Russian: Мама говорит детям успокоиться, когда они слишком громко спорят. - Mom tells the children to calm down when they argue too loudly.

So детям = to the children.
детей would be a direct object (the children as something affected by the verb), which doesn’t fit говорить in this structure.

Is детям really from дети? Why does the form change like that?

Yes. The base forms are irregular:

singular: ребёнок – a child
plural nominative: дети – children
plural dative: детям – to/for the children

So детям is simply the dative plural of дети.
This is irregular (it doesn’t follow a simple “-ы / -ам” pattern from the same stem), so it’s worth memorizing:

дети (nom. pl.)
детей (gen. pl.)
детям (dat. pl.)
детей (acc. pl.)
детьми (instr. pl.)
детях (prep. pl.)

Why is there an infinitive успокоиться after говорит детям? Why not a full clause like “чтобы они успокоились”?

Russian has a common structure:

говорить / сказать / просить / заставить / велеть + кому? + что сделать? (infinitive)

It’s very close to English “tell someone to do something”.

So:

Мама говорит детям успокоиться.
= Mom tells the children to calm down.

You can also say:

Мама говорит детям, чтобы они успокоились.

This is also correct and common.
Rough difference:

говорит детям успокоиться – compact, a bit more “instruction-like”.
говорит детям, чтобы они успокоились – slightly more explicit, more like “she says to them so that they calm down”.

Both are normal, but the infinitive pattern is extremely frequent and good to learn.

Why is it успокоиться and not успокоить or успокаиваться?

All three are related but different:

успокоиться – to calm down (oneself), to become calm
– intransitive, reflexive, perfective
успокоить (кого?) – to calm (someone) down
– transitive, non‑reflexive, perfective
успокаиваться – to be calming down, to calm down (process)
– reflexive, imperfective

In the sentence, the children must calm themselves, so we need the reflexive form успокоиться.

If you used успокоить, it would mean “to calm someone else down”:

Мама просит сына успокоить сестру.
Mom asks the son to calm his sister down.

Here that’s wrong, because we don’t want the kids to calm someone else – we want them to calm down themselves → успокоиться.

Why is the perfective успокоиться used, not imperfective успокаиваться?

In Russian, when you tell someone to do something once and reach a result, you normally use the perfective infinitive:

Лечь спать! – lie down to sleep (and be in bed)
Сесть! – sit down (and be seated)
Закрыть дверь! – close the door (and have it closed)
Успокоиться! – calm down (and be calm)

The mother wants the children to reach a calm state, not just “be in the process of calming down”.
So успокоиться (perfective) is the natural choice.

Успокаиваться would focus on ongoing process, and is much less typical as an instruction here.

Why is говорит in the present tense if this seems like a repeated/habitual action?

Russian uses the simple present tense both for:

actions happening right now
habitual / repeated actions

So:

Мама говорит детям успокоиться, когда они слишком громко спорят.
= Mom tells the children to calm down when(ever) they argue too loudly.

This describes a general rule/habit: every time they argue too loudly, she tells them to calm down.

You don’t need a special tense like “used to” or “will regularly” – the present serves that function in Russian when context shows it’s habitual.

How does когда они слишком громко спорят work? Is this “when” or “whenever”?

The clause:

когда они слишком громко спорят

literally is “when they argue too loudly,” but in context it usually means “whenever they argue too loudly.”

In Russian, когда + present tense is often used for repeated/general situations:

Когда я устаю, я пью кофе.
When(ever) I get tired, I drink coffee.

So here:

когда они слишком громко спорят
= when(ever) they argue too loudly.

If you said когда они будут спорить, that would sound more like a specific future occasion: “when they (at some future time) are arguing” – different nuance.

Why do we say слишком громко спорят here? Could we say спорят слишком громко, and is there a difference?

Both are grammatically correct:

они слишком громко спорят
они спорят слишком громко

The difference is only in slight emphasis and flow:

слишком громко спорят – sounds a bit more neutral, adverbs grouped before the verb.
спорят слишком громко – can put a little more focus on the activity “they argue,” and then qualify how.

Russian word order is flexible, and adverbs can go before or after the verb without changing the basic meaning. In everyday speech, you will hear both.

Why is они used in когда они слишком громко спорят? Could we just say когда дети слишком громко спорят?

Yes, you could say:

Мама говорит детям успокоиться, когда дети слишком громко спорят.

That’s also correct. The options are:

они – pronoun referring back to детям (they)
дети – repeating the noun (the children)

Both are natural. Using они is very typical when the group was just mentioned in another case (here, dative детям) and you want to refer back to them in nominative as the subject of the new clause.

You could even omit the subject if it’s totally clear:

…когда слишком громко спорят.

But that’s more colloquial and works best in context where it’s obvious who “they” are.

Why is there a comma before когда?

Because когда они слишком громко спорят is a subordinate clause of time.

In Russian, subordinate clauses introduced by words like когда, если, потому что, что, чтобы are almost always separated from the main clause by a comma.

Structure here:

Main clause: Мама говорит детям успокоиться
Subordinate time clause: когда они слишком громко спорят

So the comma is required:

Мама говорит детям успокоиться, когда они слишком громко спорят.

Why is it говорит (3rd person singular) and not говорят (plural)? There are “children” in the sentence.

The subject of the sentence is мама:

Мама говорит… – Mom tells… → 3rd person singular → говорит

детям is in the dative case; it’s an indirect object (the recipients of the speech), not the subject.

So:

subject: мама → говорит
indirect object: детям
infinitive object: успокоиться

If the subject were plural, you’d use говорят:

Родители говорят детям успокоиться.
The parents tell the children to calm down.

What exactly does слишком громко mean compared to очень громко?

очень громко – very loudly (high degree, neutral)
слишком громко – too loudly (excessive; more than acceptable)

So слишком usually implies a negative judgment: the volume is more than it should be, and that’s why Mom reacts.

If you said:

когда они очень громко спорят
it would mean they argue very loudly (no built‑in idea of “too much”), although in context it could still be understood as bothersome.
With слишком, the “this is too much” idea is explicit.

Is Мама говорит детям успокоиться, когда они слишком громко спорят the only way to say this, or is чтобы also possible?

You can say both:

Мама говорит детям успокоиться, когда они слишком громко спорят.
Мама говорит детям, чтобы они успокоились, когда они слишком громко спорят.

Both mean essentially the same thing.

Nuance (very slight):

Version 1 with the infinitive (успокоиться) sounds a bit more direct and compact, like a straightforward instruction.
Version 2 with чтобы + past tense (чтобы они успокоились) makes the desired result more explicit (“she says to them so that they calm down”).

In everyday speech, the infinitive version (1) is extremely common and fully natural.