W piaskownicy dzieci robią zamek z piasku i śmieją się razem.

Because w means in here, and when w expresses location, the noun usually goes into the locative case.

• base form: piaskownica = sandbox
• after w for location: w piaskownicy = in the sandbox

So:

• piaskownica = sandbox
• w piaskownicy = in the sandbox

Piaskownica means sandbox or sandpit, the place where children play with sand.

It comes from piasek = sand.
So it is literally something like a place for sand.

Dzieci is plural. It means children.

The singular is:

• dziecko = child

The plural is irregular:

• dzieci = children

That is why the verb is also plural:

• dzieci robią
• dzieci śmieją się

Robią is the 3rd person plural form of robić = to do / to make.

Because the subject is dzieci = children, you need the plural verb:

• ja robię = I do / make
• ty robisz = you do / make
• on/ona robi = he/she does / makes
• my robimy = we do / make
• wy robicie = you do / make
• oni/one robią = they do / make

So dzieci robią means the children are making / the children make.

Polish often uses robić in everyday speech where English might prefer make or build.

So:

• robić zamek z piasku = make a sandcastle
• budować zamek = build a castle

In this sentence, robią zamek z piasku sounds natural and idiomatic for children making a sandcastle.

This part has two important pieces:

• zamek = castle
• z piasku = of sand / from sand

So zamek z piasku literally means a castle made of sand.

The second z is a preposition meaning from/out of/of, and after it, piasek changes form:

• piasek = sand
• z piasku = of/from sand

That is because z in this meaning takes the genitive case.

There are actually two different prepositions that look similar in Polish:

1. w = in

   • w piaskownicy = in the sandbox

2. z = from / out of / made of

   • zamek z piasku = a castle of sand

So in this sentence:

• w piaskownicy tells you where
• z piasku tells you what material it is made from

Because after z meaning of/from/out of, Polish uses the genitive case.

So:

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

That gives:

• z piasku = of sand / from sand

Śmieją się means they are laughing or they laugh.

The basic verb is:

• śmiać się = to laugh

This verb normally comes with się. In other words, się is part of the standard dictionary form of the verb. You do not usually say just śmiać for this meaning.

Examples:

• Śmieję się = I am laughing
• Śmiejesz się = you are laughing
• Śmieją się = they are laughing

So here się is not optional; it belongs with the verb.

Yes. Polish present tense often covers both:

• they laugh
• they are laughing

So dzieci robią zamek z piasku i śmieją się razem can mean:

• the children make a sandcastle and laugh together
• the children are making a sandcastle and are laughing together

The exact English translation depends on context.

Razem means together.

So:

• śmieją się razem = they laugh together / they are laughing together

It adds the idea that the children are doing it with each other, not separately.

Putting razem at the end is very natural in Polish. It modifies the action śmieją się.

Polish word order is more flexible than English, so you could move things around for style or emphasis, but this version sounds normal and neutral:

• W piaskownicy dzieci robią zamek z piasku i śmieją się razem.

Ending with razem neatly finishes the idea laugh together.

Yes. Polish word order is fairly flexible because endings show grammatical relationships.

For example, these are all possible, with slightly different emphasis:

• Dzieci robią zamek z piasku i śmieją się razem w piaskownicy.
• W piaskownicy robią dzieci zamek z piasku i śmieją się razem.
• Zamek z piasku dzieci robią w piaskownicy i śmieją się razem.

But the original sentence sounds the most neutral and natural for a learner:

• W piaskownicy dzieci robią zamek z piasku i śmieją się razem.

Polish does not have articles like English a, an, or the.

So dzieci, zamek, and piaskownicy do not need any article word.

Depending on context, zamek z piasku could be understood as:

• a sandcastle
• the sandcastle

English has to choose; Polish often does not.

No. Zamek can mean several things, including:

• castle
• lock
• zipper

But in this sentence, zamek z piasku clearly means sandcastle, so castle is the only sensible meaning here.

A rough guide is:

• śmieją ≈ SHMYEH-yaw
• się ≈ something like shyeng, but very short and soft

A few helpful points:

• ś is a soft sh sound
• mię has a soft mye-type sound
• ją contains the nasal vowel ą
• się contains the nasal vowel ę

You do not need to pronounce the nasal vowels exactly like French-style nasal vowels; in normal speech their sound changes depending on what comes next. The most important thing at first is to recognize śmieją się as one common spoken pattern.

Sometimes, yes. Polish often drops subject pronouns, and sometimes even nouns if the context is obvious.

For example:

• Robią zamek z piasku i śmieją się razem.

This could still mean They are making a sandcastle and laughing together if the context already tells you who they are.

But keeping dzieci is clearer and more natural when you are introducing the scene.