Я дзвоню мамі, коли повертаюся додому після роботи.

Because дзвонити (to call / to phone) takes the dative case for the person you are calling.

So:

• мама = nominative, the basic dictionary form
• мамі = dative singular, meaning to/for mom

In Ukrainian, this works a bit like I call mom being understood structurally more like I phone to mom.

More examples:

• Я дзвоню другові. = I call my friend.
• Вона дзвонить сестрі. = She calls her sister.

So in your sentence, мамі is there because дзвонити + dative is the normal pattern.

Because повертатися means to return / to come back, and it normally appears with -ся.

Compare:

• повертати = to turn something back / return something
• повертатися = to return oneself, to come back

So:

• Я повертаю книжку. = I return the book.
• Я повертаюся додому. = I return home / I come back home.

The -ся here is very common in Ukrainian verbs and often marks a verb as intransitive or reflexive-like in meaning.

Because Ukrainian often uses the present tense to describe habitual or repeated actions, just like English does in sentences such as I call my mom when I get home from work.

Here:

• дзвоню = I call / I am calling
• повертаюся = I return / I am returning

But in this context, they mean something like:

• I call my mom when I come home from work
• Whenever I get back home after work, I call my mom

So this is not necessarily happening right now. It can describe a routine.

In this sentence, коли can be understood as when in a habitual sense, which often comes close to whenever in English.

Because the whole sentence describes a repeated action, the meaning is roughly:

• I call my mom when/whenever I come home from work.

So коли is not only about one specific moment. In context, it can introduce a regular situation.

Because додому means homeward / to home / home, showing movement toward home, while вдома means at home, showing location.

Compare:

• Я йду додому. = I’m going home.
• Я вдома. = I’m at home.

In your sentence, the speaker is returning home, so a word of motion is needed:

• повертаюся додому = return home

This is a very important distinction in Ukrainian.

Yes, you may hear до дому, but додому is the standard, very common, natural way to say home as a direction.

So:

• повертаюся додому = the usual choice
• повертаюся до дому = possible, but less neutral in many contexts

For learners, додому is the safest and most natural form to use when you mean going/coming home.

Because the preposition після (after) requires the genitive case.

So:

• робота = nominative
• роботи = genitive singular

That is why you get:

• після роботи = after work

More examples:

• після уроку = after the lesson
• після вечері = after dinner
• після зустрічі = after the meeting

So the form changes because of the preposition.

Ukrainian often omits possessive words like мій / моя / моїй when the relationship is obvious from context.

So:

• Я дзвоню мамі. = I call mom / I call my mom
• Я дзвоню моїй мамі. = I call my mom

Both are possible, but the version without моїй is very natural because мамі already strongly suggests my mom in normal speech.

This is similar to how English often says Mom instead of my mom, depending on context.

Дзвоню is the 1st person singular present tense form of дзвонити.

So:

• я дзвоню = I call
• ти дзвониш = you call
• він/вона дзвонить = he/she calls
• ми дзвонимо = we call
• ви дзвоните = you call
• вони дзвонять = they call

This means the speaker is talking about themself: I call.

Повертаюся is the 1st person singular present tense form of повертатися.

It breaks down like this:

• повертаю- = the present-tense verb stem
• -ся = the reflexive ending

So:

• я повертаюся = I return / I come back
• ти повертаєшся = you return
• він повертається = he returns

In your sentence, it matches я:

• Я ... повертаюся = I return

Yes. Ukrainian word order is more flexible than English word order, although some versions sound more neutral than others.

Your sentence:

• Я дзвоню мамі, коли повертаюся додому після роботи.

This is a natural, neutral order.

Other possible orders include:

• Коли повертаюся додому після роботи, я дзвоню мамі.
• Після роботи, коли повертаюся додому, я дзвоню мамі.

These all mean basically the same thing, but the emphasis changes slightly depending on what comes first.

A good rule for learners: use the original version unless you have a reason to emphasize another part.

They are very close, and in many contexts both can mean I call / I phone.

• дзвонити is very common in everyday speech
• телефонувати is also common and can sound a bit more explicitly tied to making a phone call

So you could also say:

• Я телефоную мамі, коли повертаюся додому після роботи.

That would still be natural.

However, since your sentence uses дзвоню, it is perfectly normal Ukrainian.

Most naturally, it sounds like a routine or repeated habit.

Because both verbs are in the imperfective present:

• Я дзвоню мамі...
• коли повертаюся...

the sentence usually means something like:

• I call my mom when I get home from work
• I usually call my mom after I come home from work

If you wanted to make it clearly about one future occasion, Ukrainian would normally use different forms or more context.

So the default interpretation here is habitual action.