Learn Hardcore Polish: W nocy dzieci śpią w pokoju. - At night the children sleep in the room.

The preposition w (in) + a static time expression (no movement) → w + locative: w nocy = at night / in the night.
So w noc is ungrammatical in this sense; you must say w nocy.

What exactly does w nocy mean: “at night” or “in the night”? Is there a difference?

In this sentence w nocy corresponds best to English “at night”, in a general, habitual sense:

W nocy dzieci śpią w pokoju.
At night the children sleep in the room. (generally, as a routine)

Literal translation is “in the night”, but English usually uses “at night” for generic statements. In Polish, w nocy can be used both for “at night (in general)” and “during the night (this night)”, depending on context.

Could I say nocą instead of w nocy? Is there any difference?

Yes, you can say:

Nocą dzieci śpią w pokoju.

This is also correct and means essentially the same thing: “At night the children sleep in the room.”

Differences:

w nocy – very common, neutral, everyday style.
nocą – a bit more literary, slightly more formal or poetic, but still used in normal speech.

In most everyday conversations, w nocy will sound more natural.

What is the subject of the sentence, and which case is dzieci in?

The subject is dzieci (children). It is in the nominative plural.

The full structure is:

W nocy – time expression (literally “in the night”).
dzieci – subject, nominative plural.
śpią – verb, 3rd person plural.
w pokoju – place expression (in the room), locative.

So the core sentence is Dzieci śpią – The children sleep / are sleeping.

What is the singular of dzieci, and what gender is it?

The singular is dziecko – a child.

Gender:

dziecko (sg.) – neuter.
dzieci (pl.) – grammatically treated as “non-masculine personal plural”, but in the present tense the verb form is the same as any other plural: śpią.

So:

To dziecko śpi. – The child is sleeping.
Dzieci śpią. – The children are sleeping.

Why is the verb śpią and not śpi?

The verb spać (to sleep) must agree with the subject in person and number.

on / ona / ono śpi – he / she / it sleeps
oni / one śpią – they sleep

Since dzieci is they (plural), you need the 3rd person plural form:

dzieci śpią – the children sleep / are sleeping
not dzieci śpi.

Is śpią present tense only, or can it mean future too?

Śpią is present tense of the imperfective verb spać.

In this sentence it has a habitual / general meaning:

W nocy dzieci śpią w pokoju.
At night the children sleep in the room (as a rule, usually).

To clearly express future, you’d normally say:

W nocy dzieci będą spały w pokoju.
At night the children will sleep in the room (tonight / in the future).

Context can sometimes make present sound future-like, but grammatically śpią is present.

How do you pronounce śpią and what do the marks on ś and ą mean?

Śpią is pronounced roughly like “shpyom” (with a very soft sh).

ś – a soft “sh” sound, similar to English sh in she, but a bit more “palatal” (tongue slightly higher and closer to the hard palate).
p – as in English spin.
i – helps soften ś and p, and you hear a short y/i sound.
ą (at the end of a word) – a nasal “o”; often sounds like “om” or “on” in everyday speech.

Full sentence (approximate pronunciation):
W nocy dzieci śpią w pokoju → v NOT-si DYET-chi shpyom f po-KO-yu

Why is it w pokoju and not w pokój?

Again, this is because of case.

The noun pokój (room) in the basic form is pokój (nominative).
In the locative singular, it becomes pokoju.

When w means in (location, no movement), it requires locative:

w pokoju – in the room (location)
w pokoju jest łóżko – There is a bed in the room.

If you talk about movement into the room, you usually use do + genitive:

idę do pokoju – I’m going to the room.

So w pokoju = in the room (where), do pokoju = to the room (where to).

Does pokój always mean “room”? I’ve seen it mean “peace.”

Yes, pokój has two main meanings:

room (as in a room in a house)
peace (as in absence of war)

In this sentence:

w pokoju clearly means “in the room”, because we’re talking about where the children sleep.
If we meant “in peace,” the verb and context would usually be different (e.g., żyć w pokoju – “to live in peace”).

Where are “the” and “a” in Polish? How do we know if it’s “the” children or just “children”?

Polish has no articles (no words like “a”, “an”, “the”).

The noun dzieci can mean:

children,
the children, or
even some children,

depending only on context.

So W nocy dzieci śpią w pokoju can be translated as:

At night children sleep in the room.
At night the children sleep in the room.

English must choose; Polish leaves it understood from context (for example, earlier in the conversation you may have been talking about your children, so English will naturally use “the children”).

Can I change the word order, e.g. Dzieci śpią w pokoju w nocy? Does it change the meaning?

Polish word order is quite flexible, so several variants are possible and correct:

W nocy dzieci śpią w pokoju. – neutral; emphasizes the time first.
Dzieci śpią w pokoju w nocy. – still okay; more neutral/flat listing of information.
W pokoju w nocy śpią dzieci. – puts some emphasis on dzieci at the end (it’s the children, not someone else, who sleep there at night).

The basic meaning stays the same: at night the children sleep in the room. Word order mainly affects emphasis and style, not grammar here.

Why don’t we say something like “są śpią” (are sleeping), like English “are sleeping”?

Polish does not use an auxiliary “to be” to form a present continuous tense the way English does.

English: they are sleeping → auxiliary are
- sleeping.
Polish: just śpią – this already covers both “they sleep” and “they are sleeping”, depending on context.

So:

Dzieci śpią. = The children sleep / The children are sleeping.
You do not say “dzieci są śpią” – that is incorrect.