Learn Hardcore Czech: Když je silný déšť a bouřka, naše děti zůstávají doma a dívají se na seriál. - When there is heavy rain and a storm, our children stay at home and watch a series.

Když means “when” in the sense of “whenever / if this happens, then that happens”.
In this sentence it introduces a condition or a repeated situation:

Když je silný déšť a bouřka, naše děti zůstávají doma…
= Whenever there is heavy rain and a storm, our children stay at home…

So here když is close to English “when(ever)” or “if” in general statements. It’s not used for questions like “When do they come?” – that would be kdy, not když.

Why is it “je silný déšť a bouřka” instead of just “prší”?

Both are possible, but they say slightly different things:

Je silný déšť a bouřka = There is heavy rain and a thunderstorm.
You explicitly mention rain (déšť) and thunderstorm (bouřka).
Prší = It’s raining.
This only mentions rain; it says nothing about a storm.

You could naturally say:

Když silně prší a je bouřka, naše děti zůstávají doma…
(When it rains heavily and there’s a thunderstorm…)
The book version you gave just chooses to name déšť explicitly.

Why is it “silný déšť” (literally strong rain) when in English we say “heavy rain”?

Czech and English often use different adjectives with similar nouns.

silný déšť = literally strong rain, but idiomatically heavy rain.
Other common combinations:
- silný vítr = strong wind
- silný mráz = severe frost

So silný déšť is completely natural Czech, even though the literal English translation would sound odd. You should remember silný déšť as the standard way to say “heavy rain”.

Does silný describe just déšť, or also bouřka in “silný déšť a bouřka”?

Grammatically, silný only modifies déšť:

silný déšť = heavy rain
bouřka = (just) a thunderstorm

If you wanted to say both are strong/intense, you’d normally repeat the adjective or change the structure, for example:

silný déšť a silná bouřka
silný déšť a bouřka (context might suggest both are strong, but strictly the adjective attaches only to the first noun)

Why is the verb je singular if the sentence has two things: déšť a bouřka?

Here je works like English “there is”, which in Czech stays singular even when you list more than one thing:

Je silný déšť a bouřka.
Literally: There is heavy rain and (there is) a thunderstorm.

This is a common pattern in Czech:

Je vítr a zima. – There is wind and (it is) cold.
The verb doesn’t have to switch to plural (jsou) in this kind of “there is X and Y” construction.

Why is it naše děti zůstávají, not naši děti zůstávají?

The noun dítě (child) is neuter in the singular, but its plural děti is a bit special:
in the plural, dětí behaves like a feminine plural for agreement.

So:

naše děti (not naši děti) – our children
- naše is the form for neuter sg. / feminine pl.
zůstávají – 3rd person plural verb form, matching dětí as a plural subject.

Compare:

naše auto – our car (neuter sg.)
naše děti – our children (formally feminine pl. for agreement)

Why is zůstávají (present tense) used, even though it describes a general habit?

Czech uses the present tense for general truths and repeated habits, just like English:

Když je silný déšť…, naše děti zůstávají doma.
= When(ever) there is heavy rain, our children stay at home.

Both English and Czech “present simple” cover:

regular actions: (They) zůstávají doma. – They stay at home.
general facts: Voda vaří při 100 °C. – Water boils at 100 °C.

So the present tense here is exactly what you want in Czech for a recurring situation.

What is the difference between zůstávat and zůstat?

They are an imperfective / perfective pair:

zůstávat (imperfective) – to stay, to be staying (repeated / ongoing)
- zůstávají doma – they (usually) stay at home
zůstat (perfective) – to stay (once, as a completed act)
- Zůstali doma. – They stayed at home (on one particular occasion).

In your sentence, you talk about a repeated habit, so Czech uses the imperfective: zůstávají.

Why is there se in dívají se? What is the difference between dívat se and dívat?

Dívat se is a reflexive verb and means “to watch / to look (at something)”.

dívají se na seriál – they watch a series

Without se, dívat is rare and usually part of more complex expressions. In everyday language, you almost always use the reflexive form:

dívat se na film / na seriál / na televizi – to watch a film / series / TV
dívat se z okna – to look out of the window

So se is simply part of the verb dívat se; you should treat it as a fixed unit.

Why is it “dívají se na seriál” and not just “dívají se seriál”?

With dívat se, Czech requires the preposition na before what you are looking at:

dívat se na něco – to watch / look at something

So:

dívají se na seriál – they watch a (TV) series
dívám se na film – I’m watching a film
díváme se na televizi – we’re watching TV

You cannot drop na here; dívat se seriál is incorrect.

What case is seriál in, and why?

After na with the meaning “onto / at / to watch”, Czech uses the accusative case.

na co? – on/at what?
na seriál – “at the series” (accusative singular: seriál)

A few similar examples:

na film – (to watch) a film
na hokej – (to watch) hockey
na koncert – (to go to) a concert

Why is it doma, not v domě or v domě doma?

doma means “at home” as an adverb of place.
- zůstávají doma – they stay at home.
v domě literally means “in the house” (inside the building), and sounds more physical/technical.

In everyday speech, to mean “at home” as a general location, you almost always say doma, not v domě.

What is the difference between bouřka and déšť in this sentence?

déšť = rain (the rain itself)
bouřka = thunderstorm, i.e. a storm with thunder and usually lightning

So the sentence is talking about:

silný déšť – heavy rain
bouřka – a thunderstorm, not just ordinary rain

You could also meet:

bouře – a (bigger) storm, not necessarily a classic thunderstorm.

Could I change the word order, for example: Naše děti zůstávají doma, když je silný déšť a bouřka?

Yes. Czech word order is quite flexible, and both versions are natural:

Když je silný déšť a bouřka, naše děti zůstávají doma…
Naše děti zůstávají doma, když je silný déšť a bouřka.

The meaning is the same; only the focus can shift slightly.
Czech keeps the comma between the main clause and the když-clause in both orders.

Why is there a comma before naše děti?

Because když je silný déšť a bouřka is a subordinate clause (a dependent clause) and naše děti zůstávají doma a dívají se na seriál is the main clause.

Czech punctuation rule:

You separate subordinate clauses with commas from the main clause.

So in both orders, you need a comma:

Když …, naše děti zůstávají doma…
Naše děti zůstávají doma, když …