Learn Hardcore Czech: Můj syn je nemocný a musí být doma.

Reason: it is the subject of the sentence – the person who is ill and must be at home. In Czech, the subject normally stands in the nominative case, and the verb je (is) and musí (must) agree with it in person and number (3rd person singular).

Why is nemocný ending in -ý? Could it be nemocná or nemocné?

Nemocný is an adjective meaning ill / sick. Predicate adjectives must agree with the noun they describe in gender, number, and case.

syn = masculine, singular, nominative
So the adjective must be masculine singular nominative: nemocný

Other forms would be used with different nouns:

Má dcera je nemocná. – My daughter is ill. (feminine)
Moje dítě je nemocné. – My child is ill. (neuter)

Is there any difference between je nemocný and je nemocen?

Both mean “is ill”, but they differ in style:

je nemocný – the normal, neutral, everyday form.
je nemocen – a short (nominal) form, sounds more formal, literary, or old-fashioned in modern Czech.

You’ll mostly use je nemocný in ordinary speech and writing.

What exactly does musí mean here? Is it “must” or “has to”?

Musí is the 3rd person singular present of muset (to have to / must).

In this sentence it can be translated as either:

My son is ill and must be at home.
My son is ill and has to be at home.

Czech musí does not distinguish between “must” and “has to”; context decides which English wording sounds more natural.

Why do we need být after musí? Could I just say musí doma?

After the modal verb muset you normally use an infinitive:

musí být – must be
musí jít – must go
musí spát – must sleep

So musí být doma literally means “must be at home”.

The phrase musí doma is ungrammatical; the verb muset needs another verb in the infinitive to complete the meaning.

Why is there no preposition before doma? Why not v domě or v domě instead?

Doma is a special adverb meaning “at home”. It already contains the idea of being in one’s home, so no preposition is used.

Compare:

jsem doma – I am at home
jsem v domě – I am in the house (more literal, any house)

In your sentence, doma is the natural, idiomatic choice.

What’s the difference between doma and domů?

The difference is location vs. direction:

doma – “at home” (where someone is, static location)
- Musí být doma. – He must be at home.
domů – “(to) home” (movement, going home)
- Musí jít domů. – He must go home.

In your sentence, we describe where he must be, not where he must go, so doma is correct.

Can the word order change? For example, could I say Můj syn musí být doma, protože je nemocný?

Yes. Czech word order is relatively flexible, and your alternative is completely correct:

Můj syn je nemocný a musí být doma.
– My son is ill and must be at home.
Můj syn musí být doma, protože je nemocný.
– My son must be at home because he is ill.

The meaning is almost the same; the second version makes the cause–effect relationship more explicit with protože (because).

How do I pronounce můj, nemocný, and musí?

Approximate English-based hints:

můj – [mooy]
- ů is a long u sound; j is like English y.
nemocný – [NEH-mots-nee]
- Stress is always on the first syllable in Czech: NE-moc-ný.
- c is pronounced like ts.
- ý is a long ee sound.
musí – [MOO-see] (but with short u, long í)
- u is short, í is long ee.

Proper vowel length (short vs. long) is important in Czech; it can change word meanings.

Why is there no word for “a / the” in Můj syn?

Czech generally does not use articles (no equivalents of English a/an/the).

syn can mean “son”, “a son”, or “the son” depending on context.
můj syn clearly means “my son”, so no article is needed at all.

Context and word order usually provide the specificity that English expresses with articles.