Learn Hardcore Croatian: On nije bolestan; on je zdrav.

In this sentence, on is used twice for emphasis and contrast:
On nije bolestan; on je zdrav. → He is not sick; he is healthy (he is, not someone else).

You could also say:

Nije bolestan; zdrav je.
On nije bolestan; zdrav je.

and they would still be correct and natural, just with slightly different emphasis.

What is the difference between nije and je?

Both come from the verb biti (to be) in the present tense:

je = is
nije = is not

Nije is not written as ne je; it is a single, fused form. The full present tense of biti (to be) in the 3rd person is:

(on/ona/ono) je – he/she/it is
(on/ona/ono) nije – he/she/it is not

So:

On je zdrav. – He is healthy.
On nije bolestan. – He is not sick.

Why do we say nije bolestan instead of using a separate word for not, like in English?

Croatian usually forms negation with ne + verb, written as two words:

ne radi – does not work
ne spava – does not sleep

But a few very common verbs have fused negative forms, written as one word. Biti (to be) is one of them:

jesam → nisam
jesi → nisi
je → nije
smo → nismo
ste → niste
su → nisu

So instead of ne je, you always write and say nije.
In other words: nije = ne + je, but it’s always written together.

What do bolestan and zdrav literally mean?

Both are adjectives:

bolestan – sick / ill
zdrav – healthy

They both agree with the subject on (he), which is masculine singular, so we use:

bolestan (masculine singular form)
zdrav (masculine singular form)

If the subject were feminine (ona – she), you would say:

Ona nije bolesna; ona je zdrava.

For neuter (ono – it):

Ono nije bolesno; ono je zdravo.

Why is the adjective bolestan and not bole or something shorter?

Croatian adjectives often have a longer base form with typical endings.
For bolestan, the main forms are:

Masculine: bolestan
Feminine: bolesna
Neuter: bolesno

So the -an / -na / -no part is a regular adjective ending, and the root is boles-.

Similarly, zdrav changes:

Masculine: zdrav
Feminine: zdrava
Neuter: zdravo

Can I say just On je zdrav without the first part?

Yes.

On je zdrav. – He is healthy.

is a perfectly normal, complete sentence.

The full sentence:

On nije bolestan; on je zdrav.

adds an explicit contrast: it’s not that he is sick; he is (in fact) healthy. You can think of it as:

He isn’t sick; he’s healthy.

Is the semicolon (;) important here? Could I use a comma or a period?

You don’t have to use a semicolon. All of these are possible:

On nije bolestan; on je zdrav.
On nije bolestan, on je zdrav.
On nije bolestan. On je zdrav.

A semicolon feels a bit more formal or “careful” writing, clearly showing that the two short sentences are closely related. In everyday writing, many people would just use a comma or even a period.

Is there any difference between saying nije bolestan; on je zdrav and nije bolestan, nego je zdrav?

Yes, there is a nuance.

On nije bolestan; on je zdrav.
Two sentences placed side by side. The contrast is understood from context.
On nije bolestan, nego je zdrav.
Uses nego (meaning roughly but rather / but instead), which explicitly marks a contrast or correction.

The version with nego sounds more like:

He’s not sick, but (rather) healthy.

Both are correct; nego just makes the contrast more explicit.

Can I reorder the words, like On je zdrav, nije bolestan?

Yes. Word order in Croatian is freer than in English, and you can reorder for different emphasis:

On nije bolestan; on je zdrav.
Neutral: first says what he is not, then what he is.
On je zdrav, nije bolestan.
Starts with the positive statement.
Nije bolestan; on je zdrav.
Drops the first on, keeps it in the second clause for emphasis.

All of these are grammatically correct. Which one you choose depends mostly on what you want to emphasize first.

Do I really need to repeat on twice? It feels redundant.

You don’t have to. Repeating on is a stylistic choice.

Alternatives:

On nije bolestan; zdrav je.
Nije bolestan; on je zdrav.
Nije bolestan; zdrav je.

All are valid. Repetition of on:

makes the contrast clearer,
can sound more emphatic, almost like spoken clarification:
He isn’t sick; he’s healthy (I mean him).

In neutral speech, you’d probably drop at least one on.

How do you pronounce bolestan and zdrav?

Approximate pronunciation (stressed syllable in caps):

bolestan → BO-le-stan
- bo like bo in bottle
- le like le in let
- stan like stun with a as in father
zdrav → a bit tricky at the start
- zdra like saying zdra in one cluster, close to English zdra (as in Zdravko)
- v like English v
  One syllable, something like zdrav (close to zdruhv in English approximation).

Is there any double negation here, like in some Slavic languages?

No, not in this particular sentence.

The negation is only in nije (is not). Bolestan is just an adjective, not a negation.

Croatian does use multiple negatives in some structures, for example:

Nitko nije bolestan. – Nobody is sick.
(Literally: Nobody is not sick. → but it means Nobody is sick.)

In your sentence:

On nije bolestan; on je zdrav.

there is only one negative word (nije) in the first clause, so there is no double negation issue to worry about.

Could I say this without using the verb biti (to be), like just On zdrav?

In standard Croatian, you need the verb biti in this kind of sentence:

On je zdrav. – He is healthy.

A bare On zdrav would sound unfinished or dialectal.
You can omit je only in certain special structures (headlines, slogans, or poetry), but in normal sentences, include it:

On nije bolestan; on je zdrav. ✅
On nije bolestan; on zdrav. ❌ (not standard)