Я позвоню попозже, когда вернусь домой.

Позвоню is the perfective future of позвонить. It means I will call in the sense of I will make a call / I’ll give you a ring as a single completed action.

Russian often uses:

• perfective future for a one-time completed action: позвоню
• imperfective future for an ongoing, repeated, or process-focused action: буду звонить

So in this sentence, Я позвоню попозже means:

• I’ll call a bit later
• one specific future phone call

If you said буду звонить, it would sound more like:

• I’ll be calling
• I’ll try to call
• or it would focus on the process rather than the completed act

These are an aspect pair:

• звонить = imperfective
• позвонить = perfective

Very roughly:

• звонить focuses on the process, repetition, or general activity
• позвонить focuses on one completed call

Examples:

• Я часто звоню маме. = I often call my mom.
• Я позвоню маме вечером. = I’ll call my mom in the evening.

In your sentence, the speaker means one future call, so позвоню is the natural choice.

Вернусь is the future form of вернуться, a very common perfective verb meaning to return / come back.

Why this form?

• вернуться is perfective, so вернусь means I will return
• it matches the idea of a single completed future event: first I return home, then I call

Буду возвращаться would be imperfective and would mean something more like:

• I will be returning
• I’ll be on my way back

That does not fit as well here, because the sentence means the speaker will call after arriving back home, not during the process of returning.

As for возвращусь:

• it is related to возвратиться
• it exists, but it is less common and more formal/bookish than вернусь

So когда вернусь домой is the most natural everyday phrasing.

This is a very common point of confusion for English speakers.

In Russian, after когда referring to the future, you normally still use a future-tense verb:

• когда вернусь домой = when I return home / when I get home

English usually uses the present form after when for future meaning:

• when I get home

Russian does not do that here. It uses true future meaning grammatically:

• когда я вернусь
• literally: when I will return, though you should not translate it that way in natural English

So this is normal Russian grammar, not an exception.

Because домой means to home / homeward, showing movement toward home.

Compare:

• дома = at home
• домой = home, to home
• к дому = toward the house / up to the house

In this sentence, the idea is returning to home, so домой is required:

• когда вернусь домой = when I get home

You would use дома if the speaker were already located there:

• Я буду дома в семь. = I’ll be at home at seven.

Yes, very often.

Russian frequently drops subject pronouns when the verb ending already makes the subject clear. So both are possible:

• Я позвоню попозже, когда вернусь домой.
• Позвоню попозже, когда вернусь домой.

Both mean the same thing.

Why keep Я here?

• to add emphasis
• to make the sentence a bit clearer in isolation
• because Russian often includes pronouns for style or contrast, even when they are not strictly necessary

So Я is optional here, but perfectly natural.

Both mean later, but попозже usually sounds a bit softer and more conversational.

• позже = later
• попозже = a bit later / somewhat later / later on

In everyday speech, попозже is very common when you want to sound casual and friendly:

• Я позвоню попозже. = I’ll call a bit later.

It often suggests:

• not right now
• a little later than now
• a mild, informal tone

So попозже is not drastically different in meaning, but it adds a more natural spoken feel.

Because когда вернусь домой is a subordinate clause.

The sentence has two parts:

• Я позвоню попозже = main clause
• когда вернусь домой = subordinate time clause

In Russian, subordinate clauses are normally separated by a comma, so:

• Я позвоню попозже, когда вернусь домой.

This is standard punctuation.

Yes, Russian word order is flexible, although some versions sound more natural than others.

The given sentence:

• Я позвоню попозже, когда вернусь домой.

is very natural and neutral.

Other possible orders:

• Когда вернусь домой, я позвоню попозже.
  • emphasizes the time condition first
• Попозже я позвоню, когда вернусь домой.
  • possible, but less neutral
• Я, когда вернусь домой, позвоню попозже.
  • possible in speech, but more marked

Russian word order often changes emphasis rather than core meaning. The original version is a very normal everyday way to say it.

This is because Russian verb forms often change the stem in conjugation.

Infinitive:

• вернуться = to return

1st person singular future:

• я вернусь = I will return

This is a normal conjugation pattern for this verb. The -ться of the infinitive becomes -сь in the 1st person form, and the stem changes shape:

• верну-ться
• верну-сь

Other forms are:

• я вернусь
• ты вернёшься
• он/она вернётся
• мы вернёмся
• вы вернётесь
• они вернутся

So even though вернусь may look quite different at first, it is the expected personal form of вернуться.

A helpful stress guide is:

• Я позвоню́ попо́зже, когда́ верну́сь домо́й.

Approximate pronunciation:

• ya pazva-NYOO pa-POZZ-zhe, kag-DA ver-NOOS da-MOY

Main stresses:

• позвоню́
• попо́зже
• когда́
• верну́сь
• домо́й

Stress matters in Russian, so it is worth learning these with the words.