Moji prijatelji me podržavaju kad učim hrvatski.

Because moji has to agree with prijatelji in gender, number, and case.

• prijatelji = friends, masculine plural, nominative
• so the possessive adjective my must also be masculine plural nominative
• that form is moji

A quick comparison:

• moj prijatelj = my friend
• moja prijateljica = my female friend
• moji prijatelji = my friends
• moje prijateljice = my female friends

So moji is the correct form for a group of masculine or mixed-gender friends.

Prijatelji is the nominative plural of prijatelj.

• prijatelj = friend
• prijatelji = friends

It is nominative here because it is the subject of the sentence — the people doing the action of supporting.

So in:

• Moji prijatelji me podržavaju...

the subject is my friends.

Because me is the correct unstressed accusative form of I / me.

Here is the difference:

• ja = I, used as the subject
• me = me, unstressed object form
• mene = me, stressed/full form, often used for emphasis or after prepositions

In this sentence, me is the direct object of podržavaju:

• Moji prijatelji me podržavaju = My friends support me

You would use mene if you wanted emphasis, for example:

• Moji prijatelji podržavaju mene, a ne njega. = My friends support me, not him.

Because me is a clitic, an unstressed short word, and Croatian clitics usually appear near the beginning of the clause, often in second position.

So:

• Moji prijatelji me podržavaju sounds natural

The clitic me does not usually stand by itself at the start of the sentence.

Croatian word order is flexible, but clitics follow special placement rules. That is why me appears where it does.

Podržavaju is the 3rd person plural present tense form of the verb podržavati / podržati, meaning to support.

It matches prijatelji because the subject is they:

• ja podržavam = I support
• ti podržavaš = you support
• on/ona podržava = he/she supports
• mi podržavamo = we support
• vi podržavate = you support
• oni/one podržavaju = they support

So:

• Moji prijatelji me podržavaju = My friends support me

Because the subject is plural:

• moji prijatelji = my friends

In Croatian, the verb must agree with the subject in person and number. Since friends is they, the verb must be 3rd person plural:

• prijatelj podržava = a friend supports
• prijatelji podržavaju = friends support

Because after kad meaning when, Croatian uses a finite verb, not the infinitive.

So you say:

• kad učim = when I learn / when I am learning / when I study

not:

• kad učiti ❌

The verb in the kad clause must show person and tense, so učim is used because it means I learn / I am learning / I study.

Učim is the 1st person singular present tense of učiti.

• učiti = to learn / to study
• učim = I learn / I am learning / I study

So:

• kad učim hrvatski = when I learn Croatian / when I am studying Croatian

Croatian present tense often covers meanings that English expresses with both the simple present and the present continuous.

It can mean both, depending on context.

Croatian present tense is often broader than English:

• učim hrvatski can mean I learn Croatian
• it can also mean I am learning Croatian
• in some contexts, it can also feel like I study Croatian

So in your sentence, English might translate it as:

• My friends support me when I’m learning Croatian
• or My friends support me when I study Croatian

Both are natural, depending on the situation.

Because hrvatski is the direct object here, and it refers to an understood masculine inanimate noun such as jezik.

The full phrase would be:

• učim hrvatski jezik = I am learning the Croatian language

Very often, jezik is simply left out, and hrvatski stands on its own:

• učim hrvatski

Since the understood noun is masculine inanimate, the accusative singular looks the same as the nominative singular:

• hrvatski jezik = nominative
• učim hrvatski jezik = accusative

That is why the form stays hrvatski.

Yes. Very commonly.

In Croatian, names of languages are often expressed with an adjective standing for language:

• učim hrvatski = I’m learning Croatian
• govorim engleski = I speak English
• studira francuski = he/she studies French

The full form with jezik is possible:

• učim hrvatski jezik

But in everyday speech, leaving out jezik is extremely normal.

They both mean when.

• kad = shorter, very common in everyday speech
• kada = slightly more formal or full-sounding

In this sentence, both would work:

• Moji prijatelji me podržavaju kad učim hrvatski.
• Moji prijatelji me podržavaju kada učim hrvatski.

The meaning does not really change.

Usually it sounds general or repeated:

• My friends support me when I learn/am learning Croatian
• in other words, whenever I’m studying Croatian, they support me

That comes from the use of the present tense in both clauses:

• podržavaju = they support
• učim = I learn / I’m learning

Croatian present tense can describe:

• habits
• general truths
• ongoing situations

So this sentence most naturally suggests a general situation, not just one single moment.

Yes, Croatian word order is fairly flexible, but some orders sound more natural than others because of clitic placement.

The original:

• Moji prijatelji me podržavaju kad učim hrvatski.

is very natural and neutral.

You could also hear variations such as:

• Kad učim hrvatski, moji prijatelji me podržavaju.

This changes the focus, putting when I learn Croatian first.

What usually stays important is the position of the clitic me, which must follow normal clitic rules.

No. Moji prijatelji is correct here.

• moji prijatelji = my friends

The word svoj is a reflexive possessive and is used when the possessor is the same as the subject. For example:

• Volim svoje prijatelje. = I love my friends.

There, the subject is I, and the friends belong to the subject, so svoje is natural.

But in your sentence, the subject is moji prijatelji, not I:

• Moji prijatelji me podržavaju...

So moji is exactly right.

Yes, podržavati is the imperfective verb, and that fits well here.

Croatian often has verb pairs:

• podržavati = imperfective, ongoing/repeated/general supporting
• podržati = perfective, to support once / to give support as a completed act

In this sentence, the meaning is general and ongoing:

• My friends support me when I’m learning Croatian

So the imperfective present podržavaju is the natural choice.

Likewise, učim from učiti is also imperfective, which fits the idea of an ongoing learning process.