Ko je on?

• Ko = who
• je = is
• on = he

So word-for-word, it is Who is he?

Je is the 3rd person singular present-tense form of the verb biti (to be).

So:

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

In Ko je on?, je means is.

In Serbian, the normal word order here is Ko je on?

The verb form je is a clitic, which means it usually wants to appear in the second position in the sentence. Because of that, ko comes first, and je follows it.

So:

• Ko je on? = correct
• Ko on je? = not natural/incorrect in standard usage

Not exactly. On specifically means he.

So Ko je on? means:

• Who is he?

If you want Who is that?, Serbian would usually use a different structure, such as:

• Ko je to? = Who is that?

This is a very common distinction.

A simple English-friendly guide would be:

• Ko ≈ koh
• je ≈ yeh
• on ≈ ohn

So the whole sentence sounds roughly like:

koh yeh ohn

A few notes:

• j in Serbian is pronounced like English y in yes
• e is usually a clear eh sound
• Serbian pronunciation is generally quite phonetic

In Serbian, the normal form is ko.

You may see tko in Croatian, where it is standard. Serbian uses ko much more naturally and commonly.

So for Serbian:

• Ko je on? = correct standard form

You usually need on here if you mean Who is he?

• Ko je on? = Who is he?

If you only say:

• Ko je? = Who is it?/Who is that person?

That can be a complete sentence in the right context, but it does not specifically mean Who is he? The pronoun on makes the reference clear.

Serbian does not have articles like English a/an and the.

So Serbian simply says:

• Ko je on?

There is no word corresponding to the or a here.

Not if you mean Who is he?

Onaj means something like that one or that man/person, depending on context.

So:

• Ko je on? = Who is he?
• Ko je onaj? = more like Who is that one?

That is a different meaning.

It is neutral. Ko je on? is neither especially formal nor especially informal. It is a basic everyday question.

What makes a situation formal or informal in Serbian is usually more about:

• the wider context
• tone of voice
• whether you use ti or Vi in conversation

This sentence itself is just a normal question.

You would change the pronoun:

• Ko je on? = Who is he?
• Ko je ona? = Who is she?
• Ko je ono? = Who is it?/Who is that?

The verb stays je because it is still is in the 3rd person singular.

No. Serbian does not use a helper like English do for this kind of question.

In English, compare:

• He is here.
• Is he here?

In Serbian, questions are often formed just by intonation, word order, or question words like ko (who), šta (what), gde (where), etc.

So:

• On je ovde. = He is here.
• Ko je on? = Who is he?

No extra word like do is needed.

In speech, the sentence usually has rising question intonation, especially toward the end or around the focused element.

Since ko is the question word, it often gets noticeable emphasis:

• KO je on?

But natural speech can vary depending on emotion and context:

• neutral curiosity
• surprise
• suspicion
• recognition

For example, if you are surprised, you might stress on more:

• Ko je ON? = Who is HE?

So the basic sentence stays the same, but intonation can change the nuance.