Rukoznak stoji na privatnoj ispravi.

Literally, stoji means stands and comes from stajati. But in sentences about writing, marks, signs, names, stamps, or similar things, it often means is there, appears, or is written/placed.

So in this sentence, stoji does not suggest that the rukoznak is physically standing upright. It means that the mark/signature is present on the document.

Rukoznak is in the nominative singular because it is the subject of the sentence: it is the thing that stoji.

You can see the agreement here:

• rukoznak = singular
• stoji = 3rd person singular

So the grammar matches one mark/signature stands/is present.

Because na can require different cases depending on meaning. Here it means on in the sense of location, so it takes the locative case.

Base form:

• privatna isprava = a private document

After na with location:

• na privatnoj ispravi = on a private document

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

• privatna → privatnoj
• isprava → ispravi

Because Croatian usually uses na when something is physically or visibly on a surface, including paper or a document.

A mark, signature, stamp, or note is something that appears on the document:

• na ispravi

By contrast, u ispravi would more naturally mean in the document, as part of its contents or text.

So:

• na privatnoj ispravi = on the document
• u privatnoj ispravi = in the document

The ending -oj is a normal feminine singular ending for the dative/locative form of many adjectives.

The noun isprava is feminine, and after na with location it becomes locative singular:

• isprava → ispravi

The adjective has to match it in gender, number, and case:

• privatna → privatnoj

So privatnoj is not a special word by itself; it is just the correct inflected form of privatna.

Not really. It sounds formal, specialized, or somewhat legal/administrative.

In everyday Croatian, people would usually say potpis for signature.
Rukoznak is more likely to appear in legal, historical, or official wording, and it can suggest a handwritten mark or signature-like sign.

So a learner should recognize it, but not expect to hear it very often in casual conversation.

Isprava usually means a document, especially a formal, official, or legal one.

Depending on context, it can be translated as:

• document
• certificate
• deed
• instrument
• paper

In legal language, privatna isprava often means a private document, as opposed to a public or officially issued one.

Because Croatian normally has no articles.

So:

• rukoznak can mean a rukoznak or the rukoznak
• na privatnoj ispravi can mean on a private document or on the private document

The exact meaning depends on context. Croatian speakers do not need separate words for a and the the way English does.

Yes. Croatian word order is fairly flexible.

The original sentence:

• Rukoznak stoji na privatnoj ispravi.

Another possible order:

• Na privatnoj ispravi stoji rukoznak.

Both are grammatical. The difference is mainly one of focus or emphasis:

• starting with Rukoznak focuses first on the mark/signature
• starting with Na privatnoj ispravi focuses first on the location

Yes, depending on style.

For example:

• Rukoznak je na privatnoj ispravi. = simple and neutral
• Rukoznak se nalazi na privatnoj ispravi. = more formal, is located
• Rukoznak stoji na privatnoj ispravi. = appears/is there/stands on it, often natural for visible written or marked things

So stoji is a good choice here because it fits the idea of a mark visibly appearing on a document.