Wieczorem rozmawiam z mamą przez telefon.

Rozmawiam is 1st person singular present tense of rozmawiać.

So it means I talk / I am talking.

In this sentence, depending on context, it can mean:

• I talk with my mum on the phone in the evening (habitually), or
• I’m talking with my mum on the phone this evening / in the evening (less commonly, if context makes it clear).

Polish present tense often covers both simple present and present continuous meanings that English separates.

Because rozmawiać z kimś means to talk with someone / to have a conversation with someone.

By contrast:

• mówić = to speak / to say
• rozmawiać = to talk, converse

So:

• Rozmawiam z mamą = I’m talking with my mum
• Mówię do mamy = I’m speaking to my mum
• Mówię mamie coś = I’m telling my mum something

In this sentence, the idea is a conversation, so rozmawiam is the natural choice.

Because the preposition z meaning with requires the instrumental case.

The noun mama changes like this:

• nominative: mama
• instrumental: mamą

So:

• mama = mum as the basic dictionary form
• z mamą = with mum

This is one of the most common Polish case patterns:

• z bratem = with my brother
• z siostrą = with my sister
• z kolegą = with my male friend

Mamą is the instrumental singular of mama.

For many feminine nouns ending in -a, the instrumental singular ends in -ą:

• kobieta → kobietą
• siostra → siostrą
• mama → mamą

So if you see z + noun, and it means with, you will very often get this kind of form.

Wieczorem is the normal adverbial form meaning in the evening.

The basic noun is:

• wieczór = evening

But Polish often uses a case form as an adverb of time:

• rano = in the morning
• nocą = at night / by night
• wieczorem = in the evening

So wieczorem does not mean a different thing from wieczór; it is the form used when you want to say when something happens.

Usually, no. Wieczorem is the normal and natural expression for in the evening.

W wieczór is not the standard everyday way to say it.

Use:

• wieczorem = in the evening
• rano = in the morning
• po południu = in the afternoon
• w nocy = at night

So for this sentence, Wieczorem is exactly what a learner should aim to use.

Yes, przez telefon is a very normal way to say on the phone / by phone.

Literally, przez often means through, so the phrase is something like through the telephone. But you should treat przez telefon as a fixed, natural expression.

Examples:

• Rozmawiam przez telefon. = I’m talking on the phone.
• Załatwiłem to przez telefon. = I dealt with it by phone.

So even though English uses on the phone, Polish usually uses przez telefon.

In Polish, family words like mama, tata, babcia, and so on often do not need a possessive like my when the meaning is obvious.

So:

• rozmawiam z mamą naturally means I’m talking with my mum

If you want to be more explicit, you can say:

• rozmawiam z moją mamą

That is also correct, but often unnecessary.

English uses my more often than Polish does in these situations.

Yes. Polish word order is more flexible than English word order.

The sentence:

• Wieczorem rozmawiam z mamą przez telefon.

could also appear as:

• Rozmawiam z mamą przez telefon wieczorem.
• Z mamą rozmawiam przez telefon wieczorem.
• Przez telefon rozmawiam z mamą wieczorem.

The basic meaning stays similar, but the emphasis changes.

For example:

• Wieczorem... puts focus on when
• Z mamą... puts focus on with whom
• Przez telefon... puts focus on how

The version you were given sounds natural and neutral.

Most naturally, it sounds habitual:

• In the evening, I talk with my mum on the phone.

That is because wieczorem often sets a general time frame.

But Polish present tense can also refer to a planned or context-based present/future situation. So with the right context, it could also mean something like:

• I’m talking with my mum on the phone this evening.

Without extra context, though, many learners should understand it as a general routine.

The infinitive is rozmawiać.

This verb is imperfective, which means it describes:

• an ongoing action,
• a repeated action,
• or an action without stressing completion.

That fits this sentence well.

A common perfective partner is porozmawiać, which means to have a talk / to talk for a while / to complete a conversation.

Compare:

• Rozmawiam z mamą. = I’m talking / I talk with my mum.
• Porozmawiam z mamą. = I’ll have a talk with my mum.

So the form in your sentence is the normal one for a regular or ongoing activity.

Yes. After przez, telefon is in the accusative case.

But for this noun, the accusative singular looks the same as the nominative:

• nominative: telefon
• accusative: telefon

So you do not see a spelling change here.

That is very common with many masculine inanimate nouns in Polish.

No. It is capitalized here only because it is the first word of the sentence.

In the middle of a sentence, it would normally be lowercase:

• Rozmawiam z mamą wieczorem.

So there is no special capitalization rule attached to this word itself.