Moja majka se ne brine kada moj brat kasni pet minuta, ali otac se uvek brine.

Because brinuti se is the normal Serbian verb for to worry. The se is part of the verb, not a separate word with a separate meaning here.

So:

• brinuti se = to worry
• brinem se = I worry / I am worrying
• brine se = he/she worries / is worrying

You should learn it as a whole unit: brinuti se.

Without se, brinuti usually means something more like to care for, provide for, look after in certain contexts, so the meaning changes.

In Serbian, se is a clitic, which means it usually goes in the second position of its clause, not necessarily directly before the verb.

So in:

• Moja majka se ne brine
• otac se uvek brine

the se comes early in the clause, after the first stressed element.

This is very normal Serbian word order. English speakers often expect something like majka ne se brine, but that is not correct.

In Serbian, negation with verbs is made by putting ne directly before the verb.

So:

• brine = worries
• ne brine = does not worry

That is the standard pattern:

• radi → ne radi
• zna → ne zna
• kasni → ne kasni

Moj means my, and it changes to agree with the noun’s gender and number.

Here:

• moja majka because majka is feminine singular
• moj brat because brat is masculine singular

So the possessive adjective must match the noun:

• masculine: moj
• feminine: moja
• neuter: moje

In the second clause, Serbian often leaves out a repeated possessive if it is already clear:

• ali otac se uvek brine

This means but [my] father always worries.
You could also say ali moj otac se uvek brine, but leaving out moj sounds natural because the meaning is already obvious from the context.

Because Serbian often avoids repeating words that are already understood.

The first clause already established the family context:

• Moja majka... moj brat...

So in the second clause, otac naturally means my father. Repeating moj would not be wrong, but it is unnecessary.

This kind of omission is very common in Serbian and often sounds smoother.

The present tense in Serbian can describe:

• something happening now
• a habitual action
• a general tendency

Here it expresses a general pattern or habitual behavior:

• Moja majka se ne brine...
• otac se uvek brine

This does not mean they are worrying at this exact moment. It means this is how they typically react.

English does the same thing:

• My mother doesn’t worry...
• My father always worries.

Kada means when.

In this sentence:

• kada moj brat kasni pet minuta = when my brother is five minutes late

Yes, you can also say kad. It is a shorter and very common variant.

So both are possible:

• kada moj brat kasni pet minuta
• kad moj brat kasni pet minuta

Kada can sound a little more neutral or slightly more formal; kad is very common in everyday speech.

The verb kasniti means to be late or to run late.

So:

• kasni = he/she is late

Serbian can add a time amount directly after it:

• kasni pet minuta = he is five minutes late
• kasnim deset minuta = I’m ten minutes late

This is just a standard way to express the amount of lateness.

After numbers like pet (five) and higher, Serbian uses a special numeral pattern, and the noun usually appears in the genitive plural.

So:

• pet minuta = five minutes
• deset minuta = ten minutes
• dvadeset minuta = twenty minutes

The base noun is minut.
Its genitive plural is minuta.

Compare:

• jedan minut
• dva minuta
• tri minuta
• četiri minuta
• pet minuta

Notice that 2, 3, 4 also use minuta here, although the grammar behind these numeral constructions is a bit special. For a learner, the practical thing is: pet minuta is the correct form.

Because se normally comes very early in the clause, in second position, while uvek is not a clitic.

So the natural order is:

• otac se uvek brine

not:

• otac uvek se brine

A useful beginner rule is: if a clause contains se, put it near the front, usually after the first main word or phrase.

Uvek means always.

In this sentence:

• otac se uvek brine = father always worries

It often goes before the main verb, but because se has its own special clitic position, you get:

• otac se uvek brine

That is very natural Serbian word order.

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

So:

• majka can mean mother, the mother, or a mother, depending on context
• otac can mean father, the father, or a father
• brat can mean brother, the brother, or a brother

Here the possessives make the meaning clear:

• moja majka = my mother
• moj brat = my brother

And then otac is understood as my father from context.

It is grammatically possible in some contexts, but it is not the most natural neutral word order here.

The standard, natural version is:

• Moja majka se ne brine

Because se usually goes in the second position of the clause.

So as a learner, it is best to use:

• X se ne brine
• X se brine

rather than moving se later.

Yes. In this sentence, ali means but and introduces a contrast:

• Moja majka se ne brine...
• ali otac se uvek brine.

So the contrast is:

• mother does not worry
• father always worries

It works very much like English but.

Yes:

• Moja majka se ne brine kada moj brat kasni pet minuta, ali moj otac se uvek brine.

That is also correct. It is just a bit more repetitive.

The original version sounds smoother because Serbian often omits repeated possessives when they are obvious from context.