Learn Hardcore Polish: Kiedy wujek piecze ciasto, dzieci czekają na małe ciastka. - When uncle bakes a cake, the children wait for small cookies.

Whenever uncle bakes a cake, the children wait for small cookies. (habit)
or
While uncle is baking a cake, the children are waiting. (right now)

Polish does not have a separate present continuous form like “is baking” — the simple present covers both.

What would change if we said kiedy wujek upiecze ciasto instead of kiedy wujek piecze ciasto?

Here we move from the imperfective verb piec (piecze) to the perfective verb upiec (upiecze).

Kiedy wujek piecze ciasto…
= While / whenever uncle bakes a cake… (action in progress or repeated)
Kiedy wujek upiecze ciasto…
= When uncle has finished baking the cake… (completion, one later moment in time)

So:

Kiedy wujek piecze ciasto, dzieci czekają na małe ciastka.
→ The children wait during the baking, or as a regular pattern.
Kiedy wujek upiecze ciasto, dzieci dostaną małe ciastka.
→ The children will get small cookies after he finishes baking.

Imperfective (piec) focuses on the process / habit; perfective (upiec) focuses on the completed result.

Why is there a comma after ciasto? Is it necessary?

Yes, the comma is obligatory in Polish.

Kiedy wujek piecze ciasto = a subordinate time clause (introduced by kiedy = when).
dzieci czekają na małe ciastka = the main clause.

In Polish, a subordinate clause is always separated from the main clause by a comma, regardless of the order:

Kiedy wujek piecze ciasto, dzieci czekają na małe ciastka.
Dzieci czekają na małe ciastka, kiedy wujek piecze ciasto.

Both need the comma. This is stricter than English, where the comma can sometimes be omitted.

Can we change the word order and say: Dzieci czekają na małe ciastka, kiedy wujek piecze ciasto? Does it sound natural?

Yes, that sentence is completely natural:

Dzieci czekają na małe ciastka, kiedy wujek piecze ciasto.

The meaning is essentially the same: When uncle bakes a cake, the children wait for small cookies.

Word order in Polish is relatively flexible because relationships are marked by endings and conjunctions, not by position in the sentence. Changing the order here:

does not change the basic meaning,
might slightly change what you emphasize (starting with dzieci emphasizes the children more).

Both versions are correct and common.

What exactly is the difference between ciasto, ciastko, and ciastka?

They are related but not the same:

ciasto
- grammatical gender: neuter, singular
- typical meaning: cake, dough, pastry mass (a big thing or the raw dough)
- example: Wujek piecze ciasto. – Uncle is baking a cake.
ciastko
- grammatical gender: neuter, singular
- meaning: a small cake, a cookie, a pastry – one small item
- example: Lubię to ciastko. – I like this little cake / cookie.
ciastka
- plural of ciastko (nominative or accusative)
- meaning: small cakes / cookies / pastries
- example in the sentence: małe ciastka – small cookies / little pastries

So in the sentence:

Wujek piecze ciasto – he is baking a cake / dough (one bigger thing).
Dzieci czekają na małe ciastka – the children are waiting for small cookies / little pastries (many small pieces).

Why is it na małe ciastka, not something like na małych ciastek? Which case is this?

After the verb czekać (to wait) you normally use the construction:

czekać na + accusative

So:

czekam na autobus – I’m waiting for the bus
czekamy na ciebie – we’re waiting for you
dzieci czekają na małe ciastka – the children are waiting for small cookies

Here:

na = for (in the sense "waiting for")
ciastka is accusative plural of ciastko
małe is accusative plural of the adjective mały (matching ciastka)

Forms like małych ciastek would be genitive plural, used in different structures (for example with some numbers, after some prepositions, or with quantity expressions), but not after czekać na.

What is the grammatical form of małe ciastka? Why małe, not mały or mała?

ciastka is neuter plural (nominative or accusative) of ciastko.
The adjective mały (small) must agree with the noun in gender, number, and case.

Adjective pattern for mały:

neuter singular nominative/accusative: małe ciastko (one small cookie)
neuter plural nominative/accusative: małe ciastka (small cookies)

So małe ciastka is:

gender: neuter
number: plural
case: accusative (because of na
- accusative)

That’s why it’s małe, not mały or mała.

What about dzieci – is it singular or plural, and what gender is it?

Dzieci means children and is:

plural (you can see this from the verb: dzieci czekają – 3rd person plural)
grammatically treated as neuter plural in Polish

The singular is:

dziecko – a child (neuter, singular)

Examples:

To dziecko czeka. – This child is waiting.
Te dzieci czekają. – These children are waiting.

Adjectives and pronouns used with dzieci take neuter plural forms:

małe dzieci – small children
te dzieci – these children

But the verb is just normal plural:
Dzieci czekają. (Children wait / are waiting.)

Why do we say czekać na something? Can we just say czekać ciastka or use a different preposition?

In modern standard Polish, the natural way to express to wait for something/someone is:

czekać na + accusative

So:

czekać na autobus – to wait for the bus
czekać na przyjaciela – to wait for a friend
czekać na małe ciastka – to wait for small cookies

You cannot say *czekać ciastka to mean to wait for cookies; that sounds wrong to Polish speakers.

There is also oczekiwać + genitive (more formal: to expect), e.g.:

oczekują ciastek – they expect cookies

But that changes the nuance; czekać na is the normal, everyday phrase for physically waiting.

Could we use gdy or jak instead of kiedy? Are there any differences?

All three can introduce a time clause, but they differ in style and nuance:

kiedy – the most neutral, very common in both speech and writing
- Kiedy wujek piecze ciasto, dzieci czekają na małe ciastka.
gdy – a bit more formal/literary, but also common
- Gdy wujek piecze ciasto, dzieci czekają na małe ciastka.
  This sounds perfectly correct, just slightly more “bookish”.
jak – in many regions and in colloquial speech, jak is used like kiedy
- Jak wujek piecze ciasto, dzieci czekają na małe ciastka.
  This is natural in spoken, informal Polish; in formal writing, kiedy or gdy are preferred.

So you can replace kiedy with gdy, and in informal contexts often with jak, without changing the basic meaning.

What is wujek exactly? Is there a difference between wuj and wujek?

Wujek means uncle and is the everyday, friendly word.

wujek – more common, slightly diminutive/affectionate in origin, but now the standard word for uncle
wuj – older, more formal or dialectal; you’ll see it in literature, older texts, or some fixed expressions

In most modern contexts you say:

To jest mój wujek. – This is my uncle.

In the sentence:

Kiedy wujek piecze ciasto…

wujek is in the nominative singular as the subject of the verb piecze.

Vocative (when addressing him directly) would be:

Wujku! – Uncle!

Why doesn’t Polish use a continuous form like “is baking” or “are waiting” here? How would you stress that it’s happening right now?

Polish simply does not have a separate present continuous tense. The simple present does both jobs:

Wujek piecze ciasto.
= Uncle is baking a cake. / Uncle bakes cakes.
Dzieci czekają na małe ciastka.
= The children are waiting for small cookies. / The children wait for small cookies (whenever …).

To emphasize that it’s right now, Polish usually relies on:

context:
W tej chwili wujek piecze ciasto. – Right now uncle is baking a cake.
time adverbs:
Teraz dzieci czekają na małe ciastka. – Now the children are waiting for small cookies.

But grammatically, the tense form is still just the simple present (piecze, czekają).