Moja prijateljica ponekad uči jezik na internetu.

Because moja has to agree with prijateljica.

In Serbian, possessive words like moj (my) change form to match the noun’s:

• gender
• number
• case

Here, prijateljica is:

• feminine
• singular
• nominative (the subject of the sentence)

So the correct form is moja prijateljica = my female friend.

Compare:

• moj prijatelj = my male friend
• moja prijateljica = my female friend
• moje dete = my child

Prijateljica means female friend.

It is the feminine form of:

• prijatelj = male friend / friend
• prijateljica = female friend

For an English speaker, this can feel unusual because English usually does not mark the gender of friend, but Serbian often does.

So:

• Moja prijateljica = my female friend
• Moj prijatelj = my male friend

Usually, prijateljica means female friend, not necessarily a romantic partner.

If someone wants to say girlfriend in the romantic sense, Serbian more often uses:

• devojka = girlfriend / girl

That said, context always matters, and in some situations prijateljica could be interpreted differently, but for learners the safest meaning here is female friend.

Ponekad means sometimes.

It is an adverb, so it describes how often something happens.

In this sentence:

• Moja prijateljica ponekad uči jezik na internetu.
• My friend sometimes learns/studies a language online.

Serbian word order is fairly flexible, so ponekad can move around, although some positions sound more natural than others.

For example:

• Moja prijateljica ponekad uči...
• Ponekad moja prijateljica uči...

Both are possible, but the original sentence sounds very natural.

Uči is the 3rd person singular present tense form of the verb učiti.

The subject is moja prijateljica = my female friend, which is she, so the verb must match she.

Present tense of učiti:

• ja učim = I learn / study
• ti učiš = you learn / study
• on/ona/ono uči = he/she/it learns / studies
• mi učimo = we learn / study
• vi učite = you learn / study
• oni/one/ona uče = they learn / study

So:

• moja prijateljica uči = my friend learns / studies

It can mean either learns or studies, depending on context.

That is very common with učiti.

Examples:

• Učim srpski. = I’m learning Serbian.
• Učim za ispit. = I’m studying for an exam.

In your sentence, uči jezik most naturally means something like:

• learns a language
• studies a language

Both are reasonable translations.

A useful extra point: studirati usually means to study at a university, not just to study in the general sense.

Because jezik is the direct object of uči, and here it is in the accusative singular.

The noun jezik is:

• masculine
• inanimate

For many masculine inanimate nouns in Serbian, the accusative singular is the same as the nominative singular.

So:

• nominative: jezik
• accusative: jezik

That is why the sentence says:

• uči jezik

If it were a masculine animate noun, the accusative would often look different.

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

So jezik can mean:

• a language
• the language

The exact meaning depends on context.

In this sentence, English would normally translate it as a language, because it sounds indefinite and general:

• My friend sometimes learns/studies a language online.

This is one of the big differences between English and Serbian.

Because na here means on or on/through the internet, and after na in a location meaning, Serbian uses the locative case.

So:

• internet is the base form
• na internetu = on the internet / online

This is a very common Serbian expression.

Important distinction:

• na internetu = on the internet
• u školi = in school
• na poslu = at work
• na stolu = on the table

Prepositions in Serbian often require a specific case, so learners need to memorize them together with the case they use.

Because after na with a location meaning, the noun goes into the locative singular.

The noun is:

• internet = nominative

After na, it becomes:

• na internetu

So the -u ending here marks the locative singular.

This pattern is common with many masculine and neuter nouns:

• grad → u gradu
• fakultet → na fakultetu
• internet → na internetu

Yes, very often you can.

In everyday Serbian, online is widely used, especially in casual speech.

So these can be similar:

• Moja prijateljica ponekad uči jezik na internetu.
• Moja prijateljica ponekad uči jezik online.

The version with na internetu is fully Serbian and very natural.
The version with online is also common, especially in modern speech.

No, Serbian word order is much more flexible than English word order.

The neutral version is:

• Moja prijateljica ponekad uči jezik na internetu.

But other orders are possible, depending on emphasis:

• Ponekad moja prijateljica uči jezik na internetu.
• Moja prijateljica uči jezik ponekad na internetu.
• Na internetu moja prijateljica ponekad uči jezik.

These do not all sound equally neutral, but they are possible.

English relies heavily on word order for grammar. Serbian uses endings much more, so word order is often used to change focus or emphasis rather than basic meaning.

You know it from both meaning and form.

• Moja prijateljica is in the nominative
• the verb uči agrees with it
• it is the person doing the action

So:

• Moja prijateljica = subject
• uči = verb
• jezik = direct object
• na internetu = location/medium
• ponekad = adverb of frequency

A simple breakdown is:

• Moja prijateljica — who?
• ponekad — how often?
• uči — does what?
• jezik — what?
• na internetu — where/how?

Not necessarily.

Because Serbian has no articles, jezik by itself does not tell you whether English should say:

• a language
• the language

Without more context, uči jezik usually sounds general, so learns/studies a language is the most natural translation.

If the speaker wanted to be more specific, they might say:

• uči srpski jezik = she is learning the Serbian language
• uči engleski = she is learning English

Here are the main dictionary forms:

• moj = my
• prijateljica = female friend
• ponekad = sometimes
• učiti = to learn / to study
• jezik = language / tongue
• internet = internet

In the sentence, some of them change form:

• moj → moja because it agrees with prijateljica
• učiti → uči because it is present tense, she learns/studies
• internet → internetu because it follows na in the locative

That is a very typical Serbian pattern: you learn the dictionary form, then learn how it changes in sentences.