On je svjedok.

• On = he
• je = is
• svjedok = witness

So the sentence is literally He is a witness.

Yes, Croatian often leaves out subject pronouns when they are clear from context.

However, in this sentence, on helps make the subject explicit: he. Without it, the sentence would depend much more on context.

For example:

• On je svjedok. = He is a witness.
• Svjedok je. = literally Is a witness, but in context it can mean He is a witness or She is a witness, depending on who you are talking about.

So on is used here to clearly say that the subject is he.

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

So:

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

In On je svjedok, je means is.

Croatian does not have articles like English a/an/the.

So:

• svjedok can mean a witness or the witness
• the exact meaning depends on context

That is very normal in Croatian. English needs an article here, but Croatian does not.

Yes. Svjedok is in the nominative singular.

In sentences with to be like this, Croatian normally uses the nominative for both:

• the subject: On
• the predicate noun: svjedok

So On je svjedok literally has:

• On = nominative subject
• svjedok = nominative predicate noun

Yes. On is the masculine form of he, and svjedok is a masculine noun.

So this sentence clearly refers to a male person:

• On je svjedok. = He is a witness.

If you wanted to refer clearly to a female witness, Croatian often uses:

• Ona je svjedokinja. = She is a witness.

In everyday Croatian, if you want to make it clear that the witness is female, svjedokinja is the usual form.

So learners will most often see:

• on ... svjedok
• ona ... svjedokinja

In some contexts, especially formal or legal language, masculine forms can sometimes be used more generally, but for normal learning purposes, it is best to remember:

• svjedok = male witness
• svjedokinja = female witness

Yes, Croatian word order is more flexible than English.

The neutral, straightforward order here is:

• On je svjedok.

But other orders can appear for emphasis or style, for example:

• Svjedok je on. = He is the witness / It is he who is the witness
• On svjedok? is not the normal standard sentence pattern here

A useful thing to remember is that je is a clitic, and clitics usually like to stand in the second position of the sentence or clause. That is one reason On je svjedok sounds so natural.

A rough English-friendly guide is:

• svje sounds approximately like svye
• dok sounds like dok

So svjedok is roughly SVYE-dok.

A few pronunciation notes:

• j in Croatian is pronounced like English y in yes
• e is usually a clear eh
• o is a clear o, not a diphthong

So the middle part -je- is not like English j in job.

Yes. On je svjedok. is a complete, correct, and natural sentence.

It is a very typical basic pattern in Croatian:

subject + form of biti + noun

For example:

• On je liječnik. = He is a doctor.
• Ona je učiteljica. = She is a teacher.
• On je svjedok. = He is a witness.

Context matters a lot.

If it is already obvious who he is, Croatian often drops on:

• Je svjedok is not normally used by itself as a neutral standalone sentence
• but Svjedok je can work in context

Still, when teaching beginners, On je svjedok is a good clear full sentence because it shows:

• the subject pronoun
• the verb je
• the noun svjedok

So it is both grammatical and useful for learning.