Мне спокойно, когда все дома.

Because this sentence uses a very common Russian pattern for feelings and states:

[dative pronoun/noun] + word of state

So:

• мне спокойно = I feel calm / it is calm for me
• literally: to me, [it is] calm

Russian often expresses emotions and physical/mental states this way. Compare:

• Мне холодно = I’m cold
• Мне грустно = I’m sad
• Мне хорошо = I feel good

So мне is in the dative case, because it marks the person experiencing the state.

Here спокойно is not being used like a normal English-style adjective. It is a word of state (sometimes called a predicative adverb or category of state word).

It expresses a condition or feeling:

• Мне спокойно = I feel calm
• Ему трудно = It is hard for him / He is having a hard time
• Здесь тихо = It is quiet here

So in this sentence, спокойно means something like:

• calm
• peaceful
• at ease

It describes the speaker’s internal state, not a noun.

In the present tense, Russian usually does not use быть (to be) in sentences like this.

So:

• Мне спокойно literally has no present-tense verb
• but it naturally means I feel calm / I am calm

This is normal in Russian. Compare:

• Я дома = I am at home
• Он занят = He is busy
• Мне плохо = I feel bad

If you wanted past or future, then forms of быть could appear:

• Мне было спокойно = I felt calm
• Мне будет спокойно = I will feel calm

It is a subordinate clause introduced by когда = when.

So the sentence breaks down like this:

• Мне спокойно = I feel calm
• когда все дома = when everyone is at home

Together:

• Мне спокойно, когда все дома.

The comma is used because Russian separates the main clause from the subordinate clause here, just like English often does.

Here все means everyone.

Why? Because of the context:

• все дома = everyone is at home

Everything is at home would not make much sense here.

Russian всё with ё often means everything, while все often means everyone or all. In normal writing, ё is often written as е, so context matters.

In this sentence, the natural meaning is clearly:

• all the people / everyone is home

Because дома means at home, while домой means (to) home as a direction.

Compare:

• Я дома = I am at home
• Я иду домой = I am going home

So:

• все дома = everyone is at home

If you said все домой, that would mean something like everyone, go home! or everyone is heading home, depending on context.

Here дома is an adverb, meaning at home.

That is important, because дома can also be a form of the noun дом in other contexts, but not here.

In this sentence:

• все дома does not mean something involving houses
• it means everyone is at home

So you should understand дома here as a fixed adverb of place.

Yes, you could, but the nuance is a little different.

• Мне спокойно, когда все дома = I feel calm when everyone is home
• Я спокоен, когда все дома = I am calm when everyone is home

The version with мне спокойно sounds a bit more like describing an internal feeling or state in a natural, impersonal Russian way.

The version with я спокоен is more direct and personal, using an adjective that agrees with the speaker:

• спокоен if the speaker is male
• спокойна if the speaker is female

Both are possible, but мне спокойно is very common and idiomatic.

Because this is the form Russian uses for:

• adverbs
• many words of state

It does not agree with мне or все.

So you do not change it like an adjective here. You do not say:

• мне спокойный ✘
• мне спокойная ✘

Instead, you use the fixed form:

• мне спокойно

This is the same pattern as:

• мне холодно
• мне страшно
• мне интересно

Yes. Russian word order is flexible, and you could also say:

• Когда все дома, мне спокойно.

This means the same thing, but the emphasis changes slightly.

• Мне спокойно, когда все дома.
  Focus starts with the feeling: I feel calm...
• Когда все дома, мне спокойно.
  Focus starts with the condition: When everyone is home...

Both are natural.

Yes, at least the main part Мне спокойно is an impersonal construction.

That means there is no grammatical subject like я in the main clause. Russian often does this when talking about:

• feelings
• physical conditions
• atmosphere
• possibility/necessity

Examples:

• Мне жарко = I’m hot
• Ему скучно = He is bored
• Здесь темно = It’s dark here
• Нам нельзя = We must not / It is forbidden for us

So Мне спокойно is a classic Russian impersonal pattern.

Yes. It is a very natural and common phrase meaning:

• everyone is home
• everyone is in the house

It often suggests that all the people who matter in the situation are safely back home or present. So in this sentence, the emotional idea is something like:

• I feel calm when everybody is home
• I feel at ease when the whole family/household is home

So the sentence is grammatically simple, but emotionally it can sound warm, domestic, and reassuring.