Learn Hardcore Czech: Zdraví je pro ni nejdůležitější věc a říká, že nemoc je to nejhorší. - Health is the most important thing for her and she says that illness is the worst thing.

In Czech you can’t say Zdraví je ona (Health is she) or Zdraví je jí (Health is to her). You need the preposition pro to express the idea “for her / in her view / from her perspective”:

Zdraví je pro ni nejdůležitější věc.
= Health is the most important thing for her.

So:

ona = she (subject, nominative)
jí = to her (dative)
ni = her (after preposition, accusative here: pro ni)

Why is nejdůležitější used instead of just důležitá or důležitější?

důležitý = important
důležitější = more important
nejdůležitější = the most important

Czech forms the comparative and superlative like this:

comparative: add -ější / -ší → důležitější
superlative: add nej- to the comparative → nejdůležitější

In this sentence we need the superlative because English says “the most important thing”:

nejdůležitější věc = the most important thing

Using just důležitá would mean “health is an important thing”, not the most important one.

What is the purpose of věc here? Could we just say Zdraví je pro ni nejdůležitější?

věc means thing. nejdůležitější věc = the most important thing.

You can say:

Zdraví je pro ni nejdůležitější.

This is also correct and natural; the adjective nejdůležitější is then used on its own, like “Health is (the) most important (to her).”

Differences in nuance:

Zdraví je pro ni nejdůležitější věc.
Slightly more concrete: health is the single most important thing in her life.
Zdraví je pro ni nejdůležitější.
Slightly more abstract/general: health is what matters most to her.

Both are fine; adding věc just makes the sentence a bit more explicit.

Can the word order be changed, for example to Pro ni je zdraví nejdůležitější věc or Zdraví je nejdůležitější věc pro ni?

Yes, Czech word order is quite flexible, and all of these are grammatically correct:

Zdraví je pro ni nejdůležitější věc. (neutral, very natural)
Pro ni je zdraví nejdůležitější věc. (emphasis on for her)
Zdraví je nejdůležitější věc pro ni. (slight emphasis on nejdůležitější věc)

All three mean the same in content. The differences are mostly about what you want to highlight:

Starting with Pro ni puts more focus on her perspective.
Keeping pro ni in the middle is the most neutral/common version here.

Why is there a comma before že in říká, že?

In Czech, a comma is normally used before že when it introduces a subordinate clause (indirect speech, reported content).

říká, že nemoc je to nejhorší
= she says that illness is the worst (thing)

So the rule: when že = that (introducing what someone says, thinks, feels, etc.), you put a comma before it:

Myslím, že má pravdu. – I think that he is right.
Vím, že přijde. – I know that he will come.

What exactly does že do in this sentence?

že here corresponds to English that in reported (indirect) speech.

Structure:

říká, že … = she says that …

So we have:

main clause: říká (she says)
subordinate clause: že nemoc je to nejhorší (that illness is the worst)

You could think of že as a conjunction that introduces the content of what she says, similar to English:

She says that illness is the worst.

What is the role of to in nemoc je to nejhorší? Why not just nemoc je nejhorší?

to here is a neuter pronoun roughly meaning that / that thing / it, and to nejhorší works as “the worst (thing)”.

nemoc je to nejhorší
≈ illness is the worst (thing)

Compare:

To je nejhorší. – That is the worst.
To nejhorší je nemoc. – The worst (thing) is illness.

You can say nemoc je nejhorší, but that sounds more like “illness is the worst (one)” in a comparison where the category is already clear (e.g. out of several options). Nemoc je to nejhorší feels more like a stand‑alone general statement: “illness is the worst (thing of all).”

Why is it to nejhorší and not ta nejhorší (feminine) to agree with nemoc?

Because to nejhorší is a fixed, very common pattern: to + superlative used as a general noun phrase meaning “the best / the worst / the most important (thing)”.

In this construction:

to is always neuter singular, regardless of the gender of the noun you’re talking about.
The phrase to nejhorší is not grammatically tied to nemoc by gender; it’s like saying “illness is the worst there is.”

Examples of the same pattern:

To nejlepší na tom je… – The best thing about it is…
To nejhorší už máme za sebou. – The worst (part) is behind us.
Nemoc je to nejhorší. – Illness is the worst (thing).

So ta nejhorší would be used when directly modifying a feminine noun, e.g.:

nejhorší nemoc – the worst illness
ta nejhorší nemoc – that / the very worst illness

Is nemoc in a special case here? Why not nemocí or something else?

nemoc is in the nominative singular here, because it’s the subject of the clause:

nemoc (subject) je (verb) to nejhorší (predicate)

Basic forms:

nominative singular: nemoc (an illness / illness)
instrumental singular: nemocí (with/by/through illness)
etc.

Since we’re simply saying “Illness is the worst”, the subject must be nominative: nemoc.

What are the genders and cases of zdraví, věc, and nemoc in this sentence?

In the sentence:

Zdraví je pro ni nejdůležitější věc…
…nemoc je to nejhorší.

we have:

zdraví
- gender: neuter
- case: nominative singular (subject of je)
věc
- gender: feminine
- case: nominative singular (part of the predicate: nejdůležitější věc)
nemoc
- gender: feminine
- case: nominative singular (subject of je)
ni (in pro ni)
- person: 3rd person singular feminine
- case: accusative after preposition pro

Why is říká in the present tense even though we’re reporting what she said?

Czech uses the present tense for reporting what someone says in a general, habitual, or current way, just like English:

Říká, že… – She says that… / She is saying that…

If you wanted to talk about something she said once in the past, you’d use the past tense:

Řekla, že nemoc je to nejhorší. – She said that illness is the worst.

So in the given sentence, říká suggests this is something she (often) says or still says now.

Could říká, že be replaced with říká, jak or říká, co?

Not with the same meaning:

říká, že… = she says that… (introducing a statement)
říká, jak… = she says how… (manner / way)
říká, co… = she says what… (content as a noun, often with a following verb)

In this sentence we want a full statement:

říká, že nemoc je to nejhorší
= she says that illness is the worst (thing)

Using jak or co here would either sound wrong or change the meaning:

říká, jak je nemoc nejhorší – odd / unnatural in Czech
říká, co je nejhorší – “she says what is the worst” (doesn’t specify that it’s illness unless you add it)

So že is the correct conjunction for introducing this kind of reported statement.