Moj brat sada jede bananu, jer je gladan.

Because moj must agree with the noun it describes.

• brat = brother
• It is masculine singular in the nominative
• So the correct form is moj

Compare:

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

So moj changes according to gender, number, and case.

Because bananu is in the accusative case.

In this sentence, banana is the direct object of the verb jede (eats / is eating), so Serbian uses the accusative:

• nominative: banana
• accusative: bananu

This is very common in Serbian:

• Imam bananu. = I have a banana.
• Jedem bananu. = I am eating a banana.

So the -u ending shows that the banana is the thing being eaten.

Brat is in the nominative case because it is the subject of the sentence.

• Moj brat = the person doing the action
• jede = eats / is eating

So:

• brat = nominative singular
• bananu = accusative singular

This is a very typical Serbian subject-object pattern.

Because Serbian present tense often covers both the simple present and the present continuous meanings that English separates.

So jede can mean:

• he eats
• he is eating

The context tells you which meaning is more natural. In this sentence, because of sada (now), the meaning is clearly:

• he is eating now

Without sada, it could be understood more generally depending on context.

Sada means now.

It emphasizes that the action is happening at this moment:

• Moj brat sada jede bananu. = My brother is eating a banana now.

It helps make the ongoing meaning clearer. Serbian word order is flexible, so you could also hear:

• Sada moj brat jede bananu.
• Moj brat jede sada bananu.

But Moj brat sada jede bananu is very natural.

Here je is the 3rd person singular form of the verb biti (to be), meaning is.

So:

• jer = because
• je = is
• gladan = hungry

Together:

• jer je gladan = because he is hungry

This je is not the same kind of word as the je in some past-tense forms; here it is simply the present-tense verb is.

It can, but Serbian often omits subject pronouns when they are already clear from context.

So:

• jer je gladan = because he is hungry
• jer on je gladan = because he is hungry

The second version adds on (he) for emphasis or contrast, but it is not necessary.

Since the sentence already introduced moj brat, Serbian naturally leaves out on.

Because gladan agrees with brat.

• brat is masculine singular
• so the adjective must also be masculine singular

That gives:

• gladan = hungry (masculine singular)

Compare:

• On je gladan. = He is hungry.
• Ona je gladna. = She is hungry.
• Oni su gladni. = They are hungry.

So the ending changes to match the person or thing being described.

Jer means because.

In this sentence:

• Moj brat sada jede bananu, jer je gladan.
• My brother is eating a banana now because he is hungry.

Yes, Serbian also uses other ways to express because, such as zato što. For example:

• Moj brat sada jede bananu zato što je gladan.

That is also correct.
But jer is very common and straightforward in everyday Serbian.

Because jer introduces a dependent clause giving the reason.

So the sentence has two parts:

• Moj brat sada jede bananu
• jer je gladan

In standard Serbian writing, a clause introduced by jer is usually separated by a comma.

So the comma here is normal and expected.

Because Serbian has no articles like English a and the.

So bananu can mean:

• a banana
• the banana

The exact meaning depends on context.

That is one of the big differences from English. Serbian usually relies on context, word order, and sometimes other words to show whether something is definite or indefinite.

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

The basic sentence is:

• Moj brat sada jede bananu, jer je gladan.

But other orders are possible, depending on emphasis:

• Sada moj brat jede bananu, jer je gladan.
• Moj brat jede bananu sada, jer je gladan.

However, not every order sounds equally natural in every context. The original version is a very good neutral sentence.

Jede comes from the verb jesti = to eat.

Present tense forms include:

• ja jedem = I eat / I am eating
• ti jedeš = you eat / are eating
• on/ona/ono jede = he/she/it eats / is eating
• mi jedemo = we eat / are eating
• vi jedete = you eat / are eating
• oni/one/ona jedu = they eat / are eating

So in your sentence, jede matches moj brat = he eats / is eating.