Moja prijateljica mi često šalje poruke sa posla.

Because moja has to agree with prijateljica in gender, number, and case.

• prijateljica = female friend
• It is feminine singular nominative
• So the possessive adjective must also be feminine singular nominative: moja

Compare:

• moj prijatelj = my male friend
• moja prijateljica = my female friend

It specifically means a female friend. Serbian often marks the gender of the person in the noun itself.

• prijatelj = male friend
• prijateljica = female friend

So this sentence tells you that the speaker’s friend is a woman.

mi means to me and is the dative clitic form of the pronoun.

In this sentence:

• moja prijateljica = the subject
• šalje = sends
• poruke = messages
• mi = to me

So the structure is basically:

My friend sends messages to me.

Serbian often uses the dative for the recipient of an action like sending, giving, showing, telling, and so on.

Because mi is the normal unstressed clitic form for to me.

Here are the useful forms:

• mi = short unstressed dative form
• meni = full stressed dative form
• mene = accusative/genitive form, not the right case here

So:

• Prijateljica mi šalje poruke. = neutral, everyday Serbian
• Prijateljica meni šalje poruke. = more emphatic, like to me specifically

šalje is the 3rd person singular present tense form of slati/šalje = to send.

It is 3rd person singular because the subject is moja prijateljica = my friend, which is one person.

So:

• ja šaljem = I send
• ti šalješ = you send
• on/ona šalje = he/she sends

Since prijateljica is singular, šalje is the correct form.

Because često means often, so the sentence describes a repeated, habitual action. In Serbian, that usually calls for the imperfective verb.

• šalje = is sending / sends regularly / often sends
• pošalje = sends once, completes the sending

So:

• često šalje sounds natural for something habitual
• često pošalje is much less natural in this kind of basic statement

This is a good example of Serbian aspect:

• imperfective for repeated or ongoing actions
• perfective for single completed actions

Because poruke is plural.

• poruka = message
• poruke = messages

It is also the accusative plural, because it is the direct object of šalje.

For feminine nouns like poruka, the nominative plural and accusative plural are both often -e:

• poruka = message
• poruke = messages

So šalje poruke = sends messages.

posla is genitive singular.

The preposition s/sa can take the genitive when it means from.
So:

• posao = work, job
• sa posla = from work

This is a very common expression in Serbian.

Because sa posla is the normal Serbian way to say from work in this context.

It refers to someone sending messages while at work or from the workplace/work setting.

• sa posla = from work
• iz kancelarije = from the office
• iz škole = from school, from inside the school/building/context

In many everyday situations, Serbian simply uses sa posla as the idiomatic phrase.

s and sa are the same preposition. Sa is used when it is easier to pronounce.

Serbian often prefers sa before certain consonants or consonant clusters.
That is why sa posla is standard and natural.

So:

• s prijateljem
• sa posla
• sa sestrom

You can think of sa as the easier-to-pronounce version of s.

Because mi is a clitic, and Serbian clitics usually appear very early in the sentence, often in second position.

That means they tend to come after the first stressed word or phrase.

Here:

• Moja prijateljica = first phrase
• mi = clitic, so it comes next
• često = often
• šalje poruke sa posla = the rest

So Moja prijateljica mi često šalje poruke sa posla is a very natural order.

Yes. Serbian word order is more flexible than English word order, although some versions sound more natural than others.

For example, you can also hear:

• Često mi moja prijateljica šalje poruke sa posla.
• Poruke mi moja prijateljica često šalje sa posla.

These versions shift the focus or emphasis.

But the original sentence is a very normal, neutral way to say it: Moja prijateljica mi često šalje poruke sa posla.

lj is a special Serbian sound. It is not a plain English l.

A rough English approximation is the lli sound in million for many speakers, though it is not exactly the same.

Also:

• š sounds like sh
• č sounds like ch in church
• j sounds like English y

So roughly:

• prijateljica ≈ priya-TEL-yi-tsa
• šalje ≈ SHA-lye

That is only an approximation, but it helps at first.

često is an adverb, and adverbs do not change for gender, number, or case.

So često always stays često here.

It simply tells you how often the action happens:

• često = often

Compare:

• retko = rarely
• ponekad = sometimes
• uvek = always

So this part is easy: no agreement, no case endings, just the adverb itself.