Když má můj syn teplotu, musí být doma.

Because mít normally takes a direct object in the accusative case, and teplota is a feminine noun.

• dictionary form: teplota
• accusative singular: teplotu

So:

• má teplotu = has a temperature / has a fever

In Czech, this is the normal way to say someone has a fever.

Yes, it literally means has a temperature, but in Czech this often means has a fever.

Czech speakers commonly say:

• mít teplotu = to have a fever / to be running a temperature

There is also the noun horečka = fever, but mít teplotu is extremely common in everyday speech.

Když usually means when, but in many contexts it can also feel close to if, especially in general situations.

Here:

• Když má můj syn teplotu, musí být doma.

means something like:

• When my son has a fever, he has to stay at home.
• or If my son has a fever, he must stay at home.

So když introduces a condition or repeated situation.

Because Když má můj syn teplotu is a subordinate clause, and Czech normally separates subordinate clauses with a comma.

So the structure is:

• Když má můj syn teplotu, = subordinate clause
• musí být doma. = main clause

This comma is required in standard Czech spelling.

Because Czech often leaves out subject pronouns when they are clear from the verb form or context.

Here, musí already tells you the subject is he/she/it or a formal you, and from the first clause we already know we are talking about můj syn.

So:

• musí být doma = he has to be at home

Adding on is possible, but usually only for emphasis or contrast.

This is a very natural Czech word order:

1. subordinate clause first: Když má můj syn teplotu
2. main clause second: musí být doma

Czech word order is more flexible than English, but this version sounds neutral and natural.

You could also say:

• Můj syn musí být doma, když má teplotu.

That has basically the same meaning, but the original sentence focuses first on the condition.

Because syn is a masculine noun in the nominative singular, and můj must agree with it.

So:

• můj syn = my son

Both words are in the nominative singular, because můj syn is the subject of má.

Literally, it is:

• musí = must / has to
• být = be
• doma = at home / home

So literally:

• must be at home

But in natural English, in this context, it is often best translated as:

• must stay at home
• has to stay home

Czech often uses být doma where English prefers stay home.

Doma means at home / home, and it is the normal word for being in one’s home environment.

• být doma = to be at home

By contrast:

• v domě = in the house/building

So doma is the natural choice here, because the meaning is not just physical location inside a building, but being at home rather than going out, going to school, etc.

Musí is the 3rd person singular present tense of muset = must / have to.

Conjugation of muset in the present:

• musím = I must
• musíš = you must
• musí = he/she/it must
• musíme = we must
• musíte = you plural / formal must
• musí = they must

Here it matches můj syn, which is he, so musí is correct.

Because modal verbs like muset are followed by an infinitive.

So the pattern is:

• muset + infinitive

Examples:

• Musím jít. = I have to go.
• Musí být doma. = He has to be at home.
• Musíme pracovat. = We have to work.

So být stays in the infinitive because it depends on musí.

Most naturally, it sounds like a general rule or repeated situation:

• Whenever my son has a fever, he has to stay home.

Czech often uses the present tense for this kind of habitual meaning.

In the right context, it could also refer to a current situation, but without extra context it sounds more like a general statement.