Na plaży dzieci budują zamek z piasku.

Because na often requires the locative case when it means in/on/at a place.

• basic form: plaża = beach
• locative form: plaży = on/at the beach

So:

• na plaży = on the beach

This is a very common pattern:

• na ulicy = on the street
• na stole = on the table
• na wyspie = on the island

With na, the meaning matters:

• na plaży = at/on the beach → location, so locative
• na plażę = onto/to the beach → movement toward it, so accusative

Polish word order is much more flexible than English word order. Starting with Na plaży puts the location first, a bit like saying:

• On the beach, the children are building a sandcastle.

The sentence could also be:

• Dzieci budują zamek z piasku na plaży.

That would still be correct, but the focus changes slightly. In the original sentence, the place is introduced first, which sounds natural and descriptive.

Dzieci means children, so it is plural.

It is the plural form of dziecko = child.

This is an irregular noun:

• singular: dziecko
• plural: dzieci

In this sentence, dzieci is the subject, so it is in the nominative plural.

Because the subject is dzieci = children, which is third person plural.

The verb budować means to build. Its present-tense forms include:

• buduję = I build / am building
• budujesz = you build
• buduje = he/she/it builds
• budujemy = we build
• budujecie = you all build
• budują = they build / are building

So:

• dzieci budują = the children are building

In Polish, the present tense can often translate as either build or are building, depending on context.

Because zamek is the direct object of budują, so it is in the accusative case. But for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

• nominative: zamek
• accusative: zamek

So although the case has changed in function, the form stays the same.

Compare this with a masculine animate noun, where accusative often changes:

• widzę kota = I see a cat
  nominative: kot accusative: kota

But:

• buduję zamek = I am building a castle
  nominative: zamek accusative: zamek

Yes. Zamek can mean several things in Polish, including:

• castle
• lock
• zipper

Here, because of the context z piasku and na plaży, it clearly means a sandcastle—literally a castle made of sand.

So the phrase is built like this:

• zamek z piasku = a castle of sand / a sandcastle

Because the preposition z in the sense of made of / from requires the genitive case.

• basic form: piasek = sand
• genitive form: piasku

So:

• z piasku = out of sand / made of sand

This is very common:

• dom z drewna = a house made of wood
• stół z metalu = a table made of metal
• figurka z plastiku = a figurine made of plastic

So the structure is:

• zamek z piasku = sandcastle

No. Z has more than one meaning in Polish, depending on context and case.

Very commonly:

1. z + instrumental = with

   • z mamą = with mom
   • z kolegą = with a friend

2. z + genitive = from / out of / made of

   • z Polski = from Poland
   • z piasku = made of sand

In this sentence, z means made of, so it takes the genitive:

• z piasku

Yes. Budują is present tense, and in Polish the present tense often covers both:

• children build
• children are building

In this sentence, the most natural English translation is usually:

• The children are building a sandcastle on the beach.

That is because the sentence describes an action happening in a scene right now.

Because Polish does not have articles like a/an/the.

So:

• dzieci can mean children or the children
• zamek can mean a castle, the castle, or in context a sandcastle

The exact meaning comes from context.

In this sentence, English usually needs an article, so we translate naturally as:

• The children are building a sandcastle on the beach.

But Polish does not need separate words for the or a.

Yes, very much so. Even though it is grammatically two words, learners should understand it as a fixed meaning unit:

• zamek z piasku = sandcastle

Literally it is:

• castle made of sand

Polish often expresses ideas this way instead of using a compound noun like English does. Compare:

• kubek z herbatą = a cup of tea
• sok z pomarańczy = orange juice
• posąg z brązu = a statue made of bronze

So zamek z piasku is a very natural Polish structure.

A rough pronunciation guide is:

• Na plaży dzieci budują zamek z piasku
• nah PLAH-zhih DJEH-chee boo-DOO-yon ZAH-mek s pyahs-koo

A few useful pronunciation notes:

• ż sounds like the s in measure
• dz in dzieci is part of a soft consonant cluster
• ą in budują is nasal, often sounding roughly like on at the end
• z before piasku is often pronounced like s because of the following voiceless consonant p

So in natural speech, z piasku often sounds close to s piasku.

Yes, absolutely. That is also correct Polish.

Compare:

• Na plaży dzieci budują zamek z piasku.
• Dzieci budują zamek z piasku na plaży.

Both mean the same basic thing, but the emphasis is a little different:

• Na plaży... puts the setting first
• Dzieci... puts the children first

This kind of flexibility is very normal in Polish, as long as the case endings make the relationships clear.