Ne znam koliko košta ova haljina.

Because znam is the present tense of znati (to know), and in Serbian you usually negate present-tense verbs by putting ne in front of them:

• znam = I know
• ne znam = I don’t know

Nisam is the negative form of sam (I am / I have as an auxiliary), so it is used with past forms, not with a simple present verb like znam.

Here koliko means how much.

In this sentence, it introduces an indirect question:

• Koliko košta ova haljina? = How much does this dress cost?
• Ne znam koliko košta ova haljina. = I don’t know how much this dress costs.

So koliko can mean how much or how many, depending on context.

Košta is the 3rd person singular present tense of koštati (to cost).

That matches ova haljina because haljina is singular:

• haljina košta = the dress costs

Some present-tense forms of koštati are:

• ja koštam = I cost
• ti koštaš = you cost
• on/ona/ono košta = he/she/it costs
• mi koštamo = we cost
• vi koštate = you plural/formal cost
• oni/one/ona koštaju = they cost

Because ova haljina is the subject of košta, so it stays in the nominative case.

• ova haljina = this dress (nominative)
• ovu haljinu = this dress (accusative)

In the clause koliko košta ova haljina, the dress is the thing that costs, so nominative is correct.

You would use ovu haljinu if it were a direct object, for example:

• Kupujem ovu haljinu. = I am buying this dress.

Because Serbian demonstratives like ovaj / ova / ovo (this) must agree with the noun in gender, number, and case.

Haljina is:

• feminine
• singular
• nominative here

So the correct form is ova:

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

Examples:

• ovaj kaput = this coat
• ova haljina = this dress
• ovo dete = this child

Because Serbian does not use a separate auxiliary like English do/does to form this kind of structure.

English needs:

• How much does this dress cost?

Serbian simply uses the main verb:

• Koliko košta ova haljina?

So košta already carries the meaning costs by itself. There is no extra word corresponding to English does here.

Inside the full sentence, it is an indirect question (also called an embedded question), not a direct one.

Compare:

• Koliko košta ova haljina? = direct question
• Ne znam koliko košta ova haljina. = indirect question

In Serbian, indirect questions usually keep the normal statement-like word order more easily than English does, and they do not need a word like if or whether here because koliko already introduces the clause.

Yes, absolutely. Ne znam koliko ova haljina košta is also natural.

Both are correct:

• Ne znam koliko košta ova haljina.
• Ne znam koliko ova haljina košta.

Serbian word order is more flexible than English word order. The difference is mostly about emphasis and flow, not basic meaning.

A very neutral English-like explanation would be:

• koliko košta ova haljina = emphasis slightly earlier on costs
• koliko ova haljina košta = emphasis slightly earlier on this dress

But in everyday speech, both are common.

Because da is not used before this kind of indirect question.

Serbian often uses da after verbs to introduce another clause, for example:

• Znam da dolazi. = I know that he/she is coming.

But with question words like koliko, gde, kad, zašto, kako, you normally do not use da:

• Ne znam koliko košta. = I don’t know how much it costs.
• Ne znam gde je. = I don’t know where he/she/it is.
• Ne znam kada dolazi. = I don’t know when he/she is coming.

So koliko itself introduces the clause.

The dictionary form is haljina, and it is a feminine noun.

That is why it takes feminine agreement:

• ova haljina
• lepa haljina = a beautiful dress
• nova haljina = a new dress

A common clue is that many Serbian feminine nouns end in -a, though that is not an absolute rule.

Yes. You can say:

• Ne znam koliko košta haljina.

That means I don’t know how much the dress costs or I don’t know how much a dress costs, depending on context.

Adding ova makes it specific:

• ova haljina = this dress

So ova points to a particular dress, probably one the speaker is looking at or talking about.

A rough word-for-word breakdown is:

• Ne = not
• znam = I know
• koliko = how much
• košta = costs
• ova haljina = this dress

So very literally:

• Not know-I how much costs this dress

Of course, that is not natural English, but it helps show how Serbian builds the sentence.

It is pronounced roughly KOSH-ta.

A few helpful notes:

• š sounds like sh in shoe
• košta has two syllables: koš-ta

So:

• košta ≈ KOSH-ta

And haljina is roughly HAL-yee-na, with lj sounding like a soft ly sound.

It is neutral and works in both everyday and standard language.

• Ne znam = perfectly normal in speech and writing
• ova haljina = neutral
• koliko košta = neutral

So this is a very standard, natural Serbian sentence.