Ta stvar je mala.

Because stvar is a feminine singular noun in Serbian.

The demonstrative taj / ta / to must agree with the noun in gender, number, and case:

• taj = masculine
• ta = feminine
• to = neuter

Since stvar is feminine, you say ta stvar.

Examples:

• taj grad = that city
• ta knjiga = that book
• to dete = that child

In Serbian, every noun has grammatical gender. Stvar is feminine, even though it does not end in -a, which is what many feminine nouns do.

So this is something you simply need to learn with the noun:

• stvar = feminine

You can also see it from agreement:

• ta stvar
• mala stvar

Both ta and mala are feminine forms, matching stvar.

Because adjectives must agree with the noun they describe.

Since stvar is:

• feminine
• singular
• nominative here

the adjective must also be:

• feminine
• singular
• nominative

So:

• mali = masculine
• mala = feminine
• malo = neuter

That is why you say Ta stvar je mala.

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

Here it means is.

So:

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

In Ta stvar je mala, je connects the subject and the adjective, just like is in English.

No, in standard Serbian you normally keep je in the present tense.

So you say:

• Ta stvar je mala.

Not normally:

• Ta stvar mala.

Serbian usually requires the present-tense form of to be in sentences like this.

Yes, Serbian word order is more flexible than English, but Ta stvar je mala is the most neutral and natural order.

This sentence has:

• Ta stvar = subject
• je = verb
• mala = predicate adjective

Other orders may be possible for emphasis, for example:

• Mala je ta stvar. = That thing is small. / That thing is the small one.
• Ta stvar mala je. = possible, but marked or poetic, not the normal everyday order

For learners, the safest default is:

• subject + je + adjective

Its basic meaning is that, but in real usage Serbian demonstratives can sometimes feel closer to English the, depending on context.

Still, for learning purposes, it is best to think of:

• ta = that for a feminine noun

Serbian does not have a separate word for the like English does.

So:

• stvar can mean a thing or the thing, depending on context
• ta stvar more specifically means that thing

It is in the nominative singular.

Why? Because ta stvar is the subject of the sentence, and subjects are normally in the nominative.

You can tell the whole phrase is nominative singular feminine:

• ta = nominative feminine singular
• stvar = nominative singular
• mala = nominative feminine singular

Because stvar is the subject, not an object.

In Ta stvar je mala:

• ta stvar = the thing being talked about
• mala = what is being said about it

The verb je does not force the subject into another case here. So stvar stays in the nominative.

Yes, but the meaning changes.

Serbian often distinguishes:

• ova = this
• ta = that
• ona = that over there / that more distant one

Because stvar is feminine, the forms are:

• ova stvar = this thing
• ta stvar = that thing
• ona stvar = that thing over there

So you cannot use ovo or ono directly with stvar, because those are neuter forms. You would need:

• ova stvar
• ta stvar
• ona stvar

It is pronounced approximately like stvahr, with:

• stv- all pronounced clearly together
• a rolled or tapped r
• a like the a in father

A useful breakdown is:

• stvar = stv-ar

The r in Serbian is stronger than in most English accents.

Yes. When the adjective directly describes the noun inside the noun phrase, it usually comes before the noun:

• mala stvar = a small thing

But in your sentence:

• Ta stvar je mala

the adjective comes after je because it is part of the predicate:

• The thing is small

So compare:

• mala stvar = small thing
• ta mala stvar = that small thing
• ta stvar je mala = that thing is small

Here mala is the normal feminine singular form used in this sentence.

Serbian adjectives can have short and long/definite forms, but this distinction is mainly visible in some masculine nominative singular adjectives, for example:

• mal / mali

In feminine singular, you normally just see:

• mala

So in this sentence, there is no special contrast to worry about. Mala is simply the correct form agreeing with stvar.