Mój brat kibicuje tej drużynie od dziecka, a ja dopiero zaczynam jej kibicować.

Because kibicować takes the dative case in Polish.

So:

• ta drużyna = nominative, the basic dictionary form
• tej drużynie = dative, to/for this team
• jej = dative pronoun, to it / to her, here meaning to the team

This is one of those verbs you simply learn with its required case:

• kibicować komu? czemu? = to support / root for someone/something in the dative

Examples:

• Kibicuję tej drużynie.
• Kibicuję Polsce.
• Kibicuję mojemu bratu.

So both tej drużynie and jej are there because the verb demands dative.

They are two forms of the same verb:

• kibicuje = he supports / roots for
• kibicować = to support / to root for

In the first clause, the verb is fully conjugated:

• Mój brat kibicuje... = My brother supports...

In the second clause, kibicować is an infinitive because it follows another verb:

• zaczynam kibicować = I am starting to support

This is very common in Polish:

• zaczynam czytać = I’m starting to read
• lubię pływać = I like swimming / I like to swim
• muszę pracować = I have to work

So the pattern is:

• [conjugated verb] + infinitive

Here dopiero means something like only now, just now, or not until now.

So a ja dopiero zaczynam jej kibicować suggests contrast:

• your brother has been supporting the team for a long time
• but you are only just starting now

It adds the idea that your involvement is late or recent.

Compare:

• zaczynam jej kibicować = I’m starting to support it
• dopiero zaczynam jej kibicować = I’m only just starting to support it

So dopiero gives an important nuance of just now / only recently.

Od dziecka is a fixed expression meaning since childhood or from childhood.

In this sentence it refers to mój brat:

• Mój brat kibicuje tej drużynie od dziecka = My brother has supported this team since childhood

It does not mean since he was a baby in a literal moment-by-moment sense. It is a natural idiomatic way to say that something has been true for as long as the person can remember.

You can use od dziecka in many contexts:

• Lubię muzykę od dziecka. = I’ve liked music since childhood.
• Od dziecka mieszka w Krakowie. = He has lived in Kraków since childhood.

A often links two clauses with a sense of contrast or comparison, especially when talking about two different subjects.

Here:

• Mój brat...
• a ja...

This feels like:

• My brother..., whereas I...
• My brother..., and I, on the other hand...

Why not the others?

• i = simple and, less contrastive
• ale = but, usually a stronger contradiction

In this sentence, a is the most natural choice because the speaker is gently contrasting two situations, not expressing a sharp opposition.

Yes, you can say a ja dopiero zaczynam kibicować if the meaning is clear from context. Polish often omits things that are obvious.

However, jej is very natural here because it clearly points back to tej drużynie and avoids any possible ambiguity.

So both are possible:

• a ja dopiero zaczynam jej kibicować = explicit, clear
• a ja dopiero zaczynam kibicować = shorter, fine if context is obvious

Including jej often sounds smoother when you want to keep the connection to the previously mentioned team.

It is the dative singular of ta drużyna.

Breakdown:

• ta → tej
• drużyna → drużynie

Because drużyna is a feminine noun, its dative singular form is:

• drużynie

And the matching demonstrative adjective ta changes to:

• tej

So:

• ta drużyna = this team
• tej drużynie = to this team

This is a good example of adjective + noun agreement in case, number, and gender.

Yes. They are different word types, and each changes according to its function in the sentence.

• mój = possessive adjective, my
• brat = noun, brother
• tej = demonstrative adjective, this
• drużynie = noun in dative
• jej = pronoun, to it / to her

Polish words change their forms depending on case, gender, number, and syntactic role.

For example:

• mój brat = nominative, because he is the subject
• tej drużynie = dative, because it depends on kibicować
• jej = dative pronoun replacing tej drużynie

So the changes are grammatical, not random.

No. It is very common with sports teams, but it can also mean to root for, to support, or to cheer on a person or group more generally.

Examples:

• Kibicuję tej drużynie. = I support this team.
• Kibicuję Polsce. = I’m rooting for Poland.
• Kibicuję ci. = I’m rooting for you.
• Wszyscy kibicowali zawodnikom. = Everyone cheered the athletes on.

So it is broader than just being a fan of a team, though sports is its most common context.

In this sentence, ja is included for contrast:

• Mój brat..., a ja...

That makes perfect sense, because the speaker is contrasting themselves with their brother.

But in Polish, subject pronouns are often omitted when the verb form already shows who the subject is. So in another context, you could simply say:

• Dopiero zaczynam jej kibicować.

The verb zaczynam already means I am starting, so ja is not grammatically necessary.

Here, however, ja is useful because it highlights the contrast:

• my brother vs me

The word order is natural and neutral, but Polish word order is relatively flexible.

The sentence:

• Mój brat kibicuje tej drużynie od dziecka, a ja dopiero zaczynam jej kibicować.

sounds normal because it presents:

1. subject
2. verb
3. object/complement
4. time expression

and then a parallel second clause.

You could rearrange parts for emphasis, for example:

• Tej drużynie mój brat kibicuje od dziecka...
• Ja dopiero zaczynam jej kibicować.

But changing the order changes the emphasis, not the basic meaning.

So the given order is a good standard model for learners.

For most learners, the important point is that kibicować behaves like an imperfective-style activity verb: it describes an ongoing or repeated activity, not a single completed event.

That fits the sentence well:

• brat kibicuje... od dziecka = an ongoing state/activity
• zaczynam kibicować = I’m beginning that ongoing activity

You do not normally need a perfective partner here. The sentence is about supporting/rooting for as a continuing action or habit.

So yes, aspect is relevant in the background, but the main practical takeaway is: learn kibicować as the normal verb for to support / root for, especially with the dative.

Yes, but the meaning would shift slightly.

• kibicować focuses on the action or attitude of supporting/rooting for
• być kibicem focuses more on being a fan

For example:

• Mój brat jest kibicem tej drużyny od dziecka... is possible, but a bit less direct and not quite as natural in this exact sentence.
• Mój brat kibicuje tej drużynie od dziecka... sounds very natural and dynamic.

In the second clause, zaczynam być kibicem would sound less natural than zaczynam kibicować, because starting to root for a team is more idiomatic than starting to be a fan.

So kibicować is the better choice here.