Learn Hardcore Russian: Моему другу стало спокойнее, когда он выключил телефон. - My friend felt calmer when he turned off his phone.

Кому стало/было/будет как?
- Мне стало холодно. – I got cold.
- Ей было грустно. – She was sad.
- Моему другу стало спокойнее. – My friend felt calmer.

There is no grammatical subject like “my friend” in the nominative; instead, Russian marks the experiencer of the feeling with the dative case:

кому? → моему другу (to my friend)
что стало? как стало? → стало спокойнее (became calmer)

If you said Мой друг стал спокойнее, that would be a different, personal sentence: “My friend became calmer” (as a quality or character trait), not just he felt calmer at that moment.

What exactly does стало do here? Could we leave it out?

Стало is the past tense of стать (to become) used in an impersonal way:

стало + наречие/сравнительная степень → it became (more) X, got (more) X

So моему другу стало спокойнее is literally:
“To my friend it became calmer” → My friend felt calmer / got calmer.

In the present tense, Russian often omits the verb быть:

Мне спокойно. – I feel calm.
Ему скучно. – He is bored.

But in the past, you normally need было/стало:

Мне было спокойно. – I was calm.
Мне стало спокойно. – I became calm / I calmed down.
Моему другу стало спокойнее. – My friend became calmer.

You cannot just say
✗ Моему другу спокойнее, когда он выключил телефон.
That sounds ungrammatical in Russian in the past tense; you need the verb стало (or было, казалось, etc.) to carry the tense.

Why is it стало спокойнее and not стал спокойнее?

Both forms exist, but they are different constructions:

Имpersonal:
- Моему другу стало спокойнее.
  Dative моему другу, neuter verb стало, no subject.
  Focus: his inner feeling changed at that moment.
Personal:
- Мой друг стал спокойнее.
  Nominative мой друг, masculine стал.
  Focus: he as a person is calmer now than before (often more general/lasting).

Your sentence describes how he felt after turning off the phone, so the impersonal моему другу стало спокойнее is more natural: it’s about a momentary change of state, not a permanent personality change.

Why is спокойнее used here? Why not спокойно or спокойным?

спокойно – an adverb: calmly / calm (as a state)
спокойный – an adjective: calm (describing a noun)
спокойнее – comparative: calmer / more calm

In this sentence, we talk about a change in degree: he was less calm before, and more calm after:

Моему другу стало спокойно. – My friend became calm.
Моему другу стало спокойнее. – My friend became calmer / felt calmer.

Спокойнее is the comparative form (from спокойный) and is exactly what you want for “calmer.”

Using спокойным here would be wrong:

✗ Моему другу стало спокойным. – ungrammatical

After стало in this impersonal pattern, Russian uses an adverb or comparative form, not a full adjective in the instrumental case.

So the normal patterns are:

Мне было спокойно. – I was calm.
Мне стало спокойно. – I became calm.
Мне стало спокойнее. – I became calmer.

Is спокойнее here an adjective or an adverb?

Formally, спокойнее is the comparative degree of the adjective спокойный. But in sentences like this it behaves like a predicative word, somewhere between “adjective” and “adverb.”

In practice, you don’t need to overanalyze it: in patterns like

кому стало/было как? → Ему стало спокойнее.

you can treat спокойнее as a single unit meaning “calmer / more calm” describing someone’s state.

This is the same as:

Ему стало лучше. – He felt better.
Нам стало хуже. – We felt worse.
Им стало страшнее. – They felt more scared.

Why does стало have a neuter ending if we’re talking about my (male) friend?

Because the sentence is impersonal: there is no grammatical subject.

In Russian, in impersonal constructions in the past tense, the verb is usually in neuter singular:

Стало темно. – It got dark.
Было жарко. – It was hot.
Мне стало плохо. – I felt sick.
Моему другу стало спокойнее. – My friend felt calmer.

The verb не agrees with друг at all, because друг is not the subject; моему другу is in the dative, marking the experiencer of the state.

Why is there a comma before когда?

In Russian, subordinate clauses introduced by conjunctions like когда, потому что, если, хотя are normally separated by a comma.

Your sentence has:

Main clause: Моему другу стало спокойнее
Subordinate clause of time (introduced by когда): когда он выключил телефон

So you must write:

Моему другу стало спокойнее, когда он выключил телефон.

If you switch the order, you still need a comma:

Когда он выключил телефон, моему другу стало спокойнее.

Why is выключил (perfective) used here and not выключал?

Выключил is the perfective past tense of выключить. Perfective is used for:

a single, completed action
often marking the moment something happens and causes a change

That’s exactly what is happening:

He turned off the phone once → as a result, he felt calmer.

So:

когда он выключил телефон – when he turned off the phone (finished doing it)

If you said когда он выключал телефон, that would be imperfective and would sound like:

while he was turning off the phone (process, not result)

That doesn’t fit well with стало спокойнее, which refers to the state after the action is completed. So выключил is the natural choice.

Where is the word “his” in “his phone”? Why is it just телефон?

In Russian, possessive pronouns (мой, твой, его, её, их) are often omitted when the owner is obvious from the context.

Here, он выключил телефон naturally means “he turned off his phone”, because:

We are talking about his state.
There is no reason to think the phone belongs to someone else.

You can explicitly say:

…когда он выключил свой телефон. – when he turned off his phone (his own)
…когда он выключил его телефон. – when he turned off his phone (= some other male person’s)

But in everyday Russian, simple он выключил телефон is enough and sounds more natural.

Can we change the word order to Когда он выключил телефон, моему другу стало спокойнее?

Yes, absolutely. Both orders are correct:

Моему другу стало спокойнее, когда он выключил телефон.
Когда он выключил телефон, моему другу стало спокойнее.

The difference is mostly in information flow and emphasis:

Version 1 starts with the result (he felt calmer) and then explains why.
Version 2 starts with the condition/time (when he turned off the phone) and then gives the result.

In English, this is like the difference between:

“My friend felt calmer when he turned off his phone.”
“When he turned off his phone, my friend felt calmer.”

Semantically, they are the same.

Could we say Когда мой друг выключил телефон, ему стало спокойнее instead?

Yes, that’s also correct and natural:

Когда мой друг выключил телефон, ему стало спокойнее.

Here:

мой друг is nominative (subject of выключил)
ему is dative (experiencer of стало спокойнее)

This is just a different way of referring to the same person:

First as мой друг (subject of the action),
Then as ему (dative experiencer of the feeling).

All of the following are fine and mean essentially the same thing:

Моему другу стало спокойнее, когда он выключил телефон.
Когда он выключил телефон, моему другу стало спокойнее.
Когда мой друг выключил телефон, ему стало спокойнее.

The choice is stylistic, not grammatical.