Ova devojka je moja prijateljica.

Because devojka is a feminine singular noun, and the demonstrative this has to agree with it in gender, number, and case.

In the nominative singular:

• ovaj = this (masculine)
• ova = this (feminine)
• ovo = this (neuter)

So:

• ovaj čovek = this man
• ova devojka = this girl
• ovo dete = this child

Here, devojka is feminine, so ova is the correct form.

For the same reason: moja has to agree with the noun it describes.

The noun prijateljica is feminine singular, so the possessive my must also be feminine singular:

• moj = my (masculine)
• moja = my (feminine)
• moje = my (neuter)

Examples:

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

Since prijateljica is feminine, moja prijateljica is correct.

Je means is. It is the 3rd person singular present tense of the verb biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In this sentence:

• Ova devojka je moja prijateljica.
• This girl is my friend.

So je is the Serbian equivalent of English is.

Because prijateljica is the feminine form meaning female friend, while prijatelj is the masculine form meaning male friend.

• prijatelj = male friend / friend (masculine)
• prijateljica = female friend

Since devojka refers to a female person, it is natural to say prijateljica.

Examples:

• On je moj prijatelj. = He is my friend.
• Ona je moja prijateljica. = She is my friend.

Yes. In this sentence, both are in the nominative singular.

• ova devojka is the subject
• moja prijateljica is the predicate noun after je

With the verb biti in sentences like X is Y, Serbian commonly uses the nominative for both nouns:

• Ona je doktorka. = She is a doctor.
• On je student. = He is a student.
• Ova devojka je moja prijateljica.

So even though prijateljica comes after je, it is still nominative here.

It can mean both, depending on context.

Devojka can mean:

• girl / young woman
• girlfriend

In your sentence, because it says Ova devojka je moja prijateljica = This girl is my friend, the meaning is clearly just girl / young woman.

If someone said:

• Ovo je moja devojka. that usually means:
• This is my girlfriend.

So context is very important with devojka.

Yes. Serbian word order is more flexible than English word order, because case endings and agreement give a lot of grammatical information.

The neutral, natural order here is:

• Ova devojka je moja prijateljica.

But you could also say:

• Moja prijateljica je ova devojka.

That would still be grammatical, but the emphasis changes. It sounds more like:

• My friend is this girl.

So the original order is the most normal if you are simply identifying the girl.

In standard present-tense Serbian, je is normally used here.

So:

• Ova devojka je moja prijateljica. is correct.

Leaving it out would usually sound incomplete or nonstandard in ordinary prose or careful speech.

However, in some informal styles, headlines, or special emphatic contexts, Serbian can sometimes omit forms of to be, but that is not what a learner should do at first. The safe rule is:

For X is Y in the present tense, keep je.

A rough pronunciation guide is:

Ova devojka je moja prijateljica
OH-vah deh-VOY-kah yeh MO-yah pree-yah-TELY-ee-tsah

A few helpful notes:

• j in Serbian sounds like English y in yes
• c usually sounds like ts
• lj is a special sound, somewhat like the lli in million, but smoother
• stress is not usually marked in normal writing, so learners often just imitate native audio at first

Because those are doing different jobs.

In your sentence:

• ova devojka = this girl Here ova directly describes the noun devojka, so it must match devojka in gender.

But in:

• Ovo je moja prijateljica. the word ovo means something like this or this is, pointing to a person or thing as a whole.

So:

• Ova devojka je moja prijateljica. = This girl is my friend.
• Ovo je moja prijateljica. = This is my friend.

Even though prijateljica is feminine, ovo is still used in that pointing structure.

It specifically means this girl, not a girl.

The word ova makes the noun definite and specific:

• ova devojka = this girl

If you just said:

• Devojka je moja prijateljica. that would not normally mean a girl is my friend in the way English uses the indefinite article. Serbian has no articles like a and the, so definiteness is often understood from context or shown with words like ova, ta, ona.

So ova clearly signals a specific girl: this girl.

Yes, it is natural and correct Serbian.

A native speaker might also say:

• Ona je moja prijateljica. = She is my friend.
• To je moja prijateljica. = That/This is my friend.
• Ova devojka mi je prijateljica. = This girl is a friend of mine / This girl is my friend.

But your original sentence is perfectly normal if the speaker is identifying a particular girl:

• Ova devojka je moja prijateljica.