Reszta dzieci śpi w pokoju.

Because reszta is followed by the genitive case, and the genitive plural of dzieci is also dzieci.

So:

• reszta = the rest / the remainder
• dzieci = of the children

This is a common pattern in Polish:

• reszta ludzi = the rest of the people
• reszta czasu = the rest of the time
• reszta dnia = the rest of the day

So reszta dzieci literally means the rest of the children.

Because the grammatical subject is reszta, and reszta is singular.

So Polish agrees the verb with reszta, not with dzieci:

• Reszta dzieci śpi. = The rest of the children is sleeping / are sleeping.

This can feel strange to an English speaker, because English often focuses more on the people involved than on the grammatical head of the phrase.

In standard Polish, singular agreement is the safest choice here:

• reszta dzieci śpi

You may sometimes hear plural agreement in less careful or more informal speech, but śpi is the standard form to learn.

Here dzieci is in the genitive plural.

That happens because reszta usually takes a noun in the genitive:

• reszta dzieci = the rest of the children
• reszta książek = the rest of the books
• reszta studentów = the rest of the students

A useful way to think about it is:

• reszta + genitive = the rest of ...

Because w can mean either in or into, and the case depends on whether you mean location or movement.

Here it means location: the children are sleeping in the room, not moving into it. So Polish uses the locative case:

• w pokoju = in the room

Compare:

• Jestem w pokoju. = I am in the room.
• Idę w pokój. = I am going into the room. (This is generally not the normal phrasing; Polish would usually prefer *do pokoju for movement.)*

So in this sentence, w pokoju is correct because it answers where?

Pokoju is the locative singular form of pokój.

The dictionary form is:

• pokój = room

After w when it means in and shows location, pokój changes to pokoju:

• w pokoju = in the room

This is just part of Polish case inflection. Many masculine nouns change form in the locative singular.

Yes, and it means something very similar, but the structure is different.

• Reszta dzieci śpi w pokoju. = The rest of the children are sleeping in the room.
• Pozostałe dzieci śpią w pokoju. = The remaining children are sleeping in the room.

The main differences:

1. Reszta dzieci uses a collective noun (reszta), so the verb is singular:

   • śpi

2. Pozostałe dzieci directly uses children as the subject, so the verb is plural:

   • śpią

Both are natural, but reszta dzieci emphasizes the remainder as a group, while pozostałe dzieci describes the children themselves.

Yes, you can move things around, because Polish word order is fairly flexible.

The neutral order here is:

• Reszta dzieci śpi w pokoju.

But other orders are possible, depending on emphasis:

• W pokoju śpi reszta dzieci. = It is in the room that the rest of the children are sleeping.
• Reszta dzieci w pokoju śpi. = The rest of the children are sleeping in the room. (Possible, but less neutral.)

So the basic meaning stays the same, but the focus changes.

A rough guide:

• dzieci sounds approximately like דזשե-chi / JYE-chee
• śpi sounds approximately like shpee, but with a softer ś sound than English sh

A few pronunciation points:

• dz in dzieci is part of a single consonant sound
• ci here sounds like a soft chi
• ś is a soft consonant, made with the tongue closer to the hard palate than English sh
• Polish stress is usually on the second-to-last syllable, so:
  • DZIE-ci
  • ŚPI is only one syllable
  • po-KO-ju

Because Polish has no articles like English the or a/an.

So:

• reszta dzieci can mean the rest of the children
• w pokoju can mean in the room

Polish usually relies on context instead of articles. That is completely normal.

It can mean either, depending on context.

Polish often uses the present tense where English might use either:

• sleeps
• is sleeping

So:

• Reszta dzieci śpi w pokoju.

could mean:

• The rest of the children are sleeping in the room.
• The rest of the children sleep in the room.

In most everyday situations, this sentence will usually be understood as are sleeping, because that is the most natural meaning with sleep in this context.