Ova stvar je u mojoj torbi.

Because stvar is a feminine singular noun in Serbian.

The demonstrative ovaj / ova / ovo changes to match the noun’s gender and number:

• ovaj = masculine singular
• ova = feminine singular
• ovo = neuter singular

So:

• ova stvar = this thing

Since stvar is feminine, ova is the correct form.

Stvar is in the nominative singular.

In this sentence, ova stvar is the subject of the sentence:

• Ova stvar = the thing that is somewhere

In Serbian, the subject is normally in the nominative case.

Because the preposition u here means in, expressing location, and that requires the locative case.

The base form is:

• moja torba = my bag / my purse

But after u when it means in or inside, you use the locative:

• u mojoj torbi = in my bag

Both words change because the adjective/pronoun must agree with the noun:

• moja → mojoj
• torba → torbi

Not always.

With u, the case depends on the meaning:

• u + locative = location, being in somewhere
• u + accusative = movement into somewhere

So:

• Ova stvar je u mojoj torbi. = This thing is in my bag. → locative
• Stavljam ovu stvar u moju torbu. = I am putting this thing into my bag. → accusative

This location vs. direction distinction is very important in Serbian.

Je is the 3rd person singular present form of the verb biti = to be.

So:

• je = is

The sentence structure is:

• Ova stvar = this thing
• je = is
• u mojoj torbi = in my bag

Serbian often omits subject pronouns like on, ona, ono, but the verb to be itself is still usually needed in sentences like this.

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

Neutral order:

• Ova stvar je u mojoj torbi.

Other possible orders:

• U mojoj torbi je ova stvar.
• Ova stvar u mojoj torbi je. — possible, but less neutral and more marked

The most natural version for a basic statement is still:

• Ova stvar je u mojoj torbi.

Word order often changes for emphasis, contrast, or style.

Stvar usually means thing, but it can also mean:

• matter
• issue
• affair
• fact, in some expressions

In this sentence, it simply means a physical thing or object.

So ova stvar is a very general expression, like English this thing.

Because mojoj is the feminine singular locative/dative form of moja.

The possessive moj / moja / moje changes like an adjective to match the noun in gender, number, and case.

With torba:

• nominative: moja torba
• locative: u mojoj torbi

So the ending -oj shows that the word is agreeing with a feminine singular noun in the required case.

Usually yes, but it can cover a few English words depending on context, such as:

• bag
• handbag
• purse
• sometimes even schoolbag or similar types of bags

So u mojoj torbi could mean:

• in my bag
• in my purse

The exact English word depends on the situation.

You often know from two things:

1. The preposition

   • u with location usually takes the locative

2. The ending

   • many feminine nouns ending in -a in nominative singular change to -i in locative singular

So:

• torba → torbi

That same ending also appears in some other cases, so the preposition and meaning help you identify it correctly.

Yes, grammatically you can.

• Stvar je u mojoj torbi. = The thing is in my bag / A thing is in my bag, depending on context

Adding ova makes it specifically:

• this thing

So ova points to a particular object near the speaker or relevant in the conversation.

A simple pronunciation guide is:

• Ova ≈ OH-vah
• stvar ≈ stvahr
• je ≈ yeh
• u ≈ oo
• mojoj ≈ MO-yoy
• torbi ≈ TOR-bee

A rough full pronunciation:

• OH-vah stvahr yeh oo MO-yoy TOR-bee

A few notes:

• j in Serbian is pronounced like English y in yes
• r is trilled or tapped
• stress is not usually marked in normal spelling, so learners often pick it up by listening

Yes: both ova and mojoj must agree with the nouns they describe.

Agreement in Serbian means matching the noun in:

• gender
• number
• case

So here:

• ova agrees with stvar
  • feminine, singular, nominative
• mojoj agrees with torbi
  • feminine, singular, locative

This kind of agreement is a core feature of Serbian grammar.