Na korytarzu czeka siedemnaście dzieci z plecakami.

Because Polish normally says na korytarzu for in/on the corridor / in the hallway.

Even though English uses in, Polish often uses na with places like this:

• na korytarzu = in the hallway
• na dworcu = at the station
• na poczcie = at the post office

So this is something you usually just learn as the standard expression. W korytarzu is much less natural here.

It is locative singular.

The preposition na can take different cases depending on meaning:

• na + accusative for movement onto/toward something
• na + locative for location

Here there is no movement, just location, so we use the locative:

• korytarz = corridor, hallway
• na korytarzu = in the hallway

This is a very common Polish number rule.

With numbers 5 and above, the noun phrase is treated differently from normal plural subjects, and the verb is usually in the 3rd person singular form:

• pięć osób czeka = five people are waiting
• dziesięć kotów śpi = ten cats are sleeping
• siedemnaście dzieci czeka = seventeen children are waiting

So even though the meaning is plural, the verb appears in singular form.

Because after numbers 5 and higher, the counted noun usually goes into the genitive plural.

So:

• dziecko = child
• dzieci = children

After siedemnaście, Polish requires the genitive plural. In this particular word, the genitive plural looks the same as the nominative plural:

• nominative plural: dzieci
• genitive plural: dzieci

So the form stays dzieci, but grammatically it is functioning as genitive plural after the number.

It is plural.

The singular is:

• dziecko = child

The plural is irregular:

• dzieci = children

This is like English child → children: the plural does not look like a regular ending.

Because the preposition z meaning with takes the instrumental case.

So:

• plecak = backpack
• instrumental plural: plecakami
• z plecakami = with backpacks

Compare:

• z kolegą = with a friend
• z dziećmi = with children
• z plecakami = with backpacks

Be careful: z can mean different things.

• z + instrumental = with
• z + genitive = from / out of

So here it means with backpacks, not from backpacks.

Usually yes, or at least that they are accompanied by backpacks / have backpacks with them.

In natural English, you would often understand this as:

• seventeen children with backpacks are waiting in the hallway

So z plecakami describes the children.

Yes. Polish word order is much freer than English word order.

This sentence:

• Na korytarzu czeka siedemnaście dzieci z plecakami.

could also be reordered, for example:

• Siedemnaście dzieci z plecakami czeka na korytarzu.

Both are grammatical. The difference is mainly in focus or style.

Starting with Na korytarzu puts the location first, as if answering:

• Where is this happening?
  Na korytarzu...

So the original sentence sounds natural if the hallway is the main setting.

Because Polish has no articles.

English distinguishes:

• a hallway
• the hallway

Polish does not do that with separate words. So:

• na korytarzu can mean in a hallway or in the hallway
• dzieci can mean children or the children

The exact meaning depends on context.

It can mean either, depending on context.

Polish present tense often covers both:

• waits
• is waiting / are waiting

So:

• czeka can mean is waiting
• in this sentence, czeka siedemnaście dzieci means seventeen children are waiting

Polish does not need a separate form like English is waiting here.

The dictionary form is czekać, which means to wait.

Here:

• czekać = infinitive
• czeka = 3rd person singular present

Examples:

• Czekam = I am waiting
• Czekasz = you are waiting
• Czeka = he/she/it is waiting

And in sentences with numbers like siedemnaście dzieci, Polish still uses that singular czeka.

Yes. With 2, 3, 4, Polish behaves differently.

For example:

• Dwoje dzieci czeka = Two children are waiting
• Trzy dzieci czekają = Three children are waiting
• Cztery dzieci czekają = Four children are waiting

But:

• Pięć dzieci czeka = Five children are waiting
• Siedemnaście dzieci czeka = Seventeen children are waiting

So one important difference is:

• 2–4 often take a plural verb
• 5+ usually take a singular verb

Because czekają would normally agree with a regular plural subject, but siedemnaście dzieci follows the special numeral pattern used with 5 and above.

So:

• dzieci czekają = the children are waiting
• trzy dzieci czekają = three children are waiting
• siedemnaście dzieci czeka = seventeen children are waiting

This difference is one of the trickier parts of Polish grammar for English speakers.