Moj brat zna tu pesmu.

Because moj must agree with the noun it describes.

• brat is masculine singular
• so the correct form is moj

Compare:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

So moj is the masculine singular form of my.

Because brat is the subject of the sentence, and subjects are normally in the nominative case.

Here:

• moj brat = the subject
• zna = the verb
• tu pesmu = the direct object

So brat stays in the nominative: brat.

Zna is the 3rd person singular present tense of znati.

So:

• ja znam = I know
• ti znaš = you know
• on/ona zna = he/she knows

In this sentence, zna agrees with moj brat, which is he, so zna means knows.

With a song, znati usually means:

• to know the song
• to be familiar with it
• sometimes to know how it goes, possibly even by heart depending on context

Because tu pesmu is the direct object, and in Serbian direct objects often take the accusative case.

The nominative form is:

• ta pesma = that song

But after a verb like zna, you need the accusative:

• tu pesmu

Both words change:

• ta → tu
• pesma → pesmu

This is very common with feminine nouns ending in -a:

• nominative: pesma
• accusative: pesmu

Because pesma is a feminine noun, and feminine nouns ending in -a usually change to -u in the accusative singular.

So:

• nominative: pesma
• accusative: pesmu

Since the song is the thing being known, it is the direct object, so Serbian uses pesmu.

Tu is the feminine accusative singular form of taj, a demonstrative word often translated as that.

Here it means:

• tu pesmu = that song

In real usage, Serbian demonstratives do not always match English this/that perfectly. Depending on context, tone, and situation, English might sometimes translate it differently. But the most straightforward translation here is that song.

Because Serbian does not have articles like English a and the.

So Serbian often expresses definiteness through:

• context
• word order
• demonstratives like taj / ta / to

In this sentence, tu pesmu already makes the noun specific, so it naturally feels like that song in English.

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

The neutral order here is:

• Moj brat zna tu pesmu.

But you could also say:

• Tu pesmu zna moj brat.
• Moj brat tu pesmu zna.

These versions are grammatical, but they shift the focus or emphasis.

For example:

• Tu pesmu zna moj brat. may emphasize that my brother is the one who knows that song.
• Moj brat zna tu pesmu. is the most neutral statement.

Not by itself.

Moj brat zna tu pesmu usually means my brother knows that song.

If you want to say knows how to sing that song, Serbian would normally say something more explicit, for example:

• Moj brat zna da peva tu pesmu. = My brother knows how to sing that song.
• Moj brat ume da peva tu pesmu. = My brother can sing that song / knows how to sing it.

So znati on its own here means knowledge or familiarity with the song, not specifically the ability to perform it.

A rough pronunciation guide for an English speaker is:

• moj ≈ moy
• brat ≈ braht
• zna ≈ zna
• tu ≈ too
• pesmu ≈ PESS-moo

A more connected rough version:

• Moy braht zna too PESS-moo

A few helpful notes:

• j in Serbian sounds like English y in yes
• r is rolled or tapped
• a, e, u are pure vowels, not diphthongs like in English

Usually, no.

In Serbian:

• znati = to know, be familiar with, know information, know a song, know a language
• poznavati / poznati = to know a person, place, or something through acquaintance

So:

• Znam tu pesmu. = I know that song.
• Poznajem tog čoveka. = I know that man.

For a song, znati is the natural verb, not poznavati.