W tej piaskownicy dobrze się bawi, bo piasek jest suchy i ciepły.

Because w here means in, and after w Polish normally uses the locative case for location.

So:

• dictionary form: ta piaskownica = this sandbox
• after w: w tej piaskownicy = in this sandbox

Both words change form:

• ta → tej
• piaskownica → piaskownicy

They both have to match in case, gender, and number.

It is locative singular.

Why?

• the preposition w often takes the locative when it means in/inside
• piaskownica is a feminine singular noun
• its locative singular form is piaskownicy

So:

• piaskownica = nominative
• piaskownicy = locative

Tej is the feminine singular locative form of ta.

The basic forms are:

• ten = this, masculine
• ta = this, feminine
• to = this, neuter

Because piaskownica is feminine, and because w requires the locative here, ta changes to tej.

So:

• ta piaskownica = this sandbox
• w tej piaskownicy = in this sandbox

Polish often omits subject pronouns when the verb form already gives enough information.

Bawi is 3rd person singular, so the sentence implies a subject such as:

• he
• she
• the child
• another singular person already known from context

English usually needs the subject stated, but Polish often leaves it out unless it needs emphasis or clarification.

For emphasis, you could say:

• On dobrze się bawi... = He is having fun...
• Ona dobrze się bawi... = She is having fun...

Not usually. In this sentence, dobrze się bawi means is having a good time, is having fun, or is enjoying himself/herself.

Although dobrze literally means well, the expression dobrze się bawić is a very common phrase meaning:

• to have fun
• to enjoy oneself

So a more natural English interpretation is not plays well, but is having fun.

Because bawić się and grać mean different kinds of play.

• bawić się = to play, have fun, amuse oneself
• grać = to play a game, play music, or perform in a play

In a sandbox, a child normally bawi się, not gra.

Examples:

• Dziecko bawi się w piaskownicy. = The child is playing in the sandbox.
• Dziecko gra w szachy. = The child is playing chess.
• Ona gra na pianinie. = She plays the piano.

Się is part of the verb bawić się.

It often has a reflexive flavor, but you should usually learn bawić się as a whole expression, not as bawić plus a separately translatable się every time.

In this sentence:

• bawić się = to have fun / to play

Very often, się does not translate neatly into English. It is just part of how the Polish verb works.

Because się is a short unstressed word, and in Polish it often appears after the first stressed element or near the verb, rather than at the very beginning.

So dobrze się bawi is natural word order.

You may also hear other orders, depending on emphasis, for example:

• Dobrze się bawi w tej piaskownicy.
• W tej piaskownicy dobrze się bawi.
• Bawi się dobrze w tej piaskownicy.

These are all understandable, but the emphasis changes slightly.

Because in Polish, bo normally introduces a clause giving a reason, and it is preceded by a comma.

So:

• ..., bo piasek jest suchy i ciepły. = ..., because the sand is dry and warm.

This comma is standard Polish punctuation.

Yes.

• bo = because, common and natural in everyday speech
• ponieważ = because/since, usually a bit more formal or written

So this is also correct:

• W tej piaskownicy dobrze się bawi, ponieważ piasek jest suchy i ciepły.

The meaning stays basically the same, but bo sounds more conversational.

Because they describe piasek, which is:

• masculine
• singular
• nominative

After jest, predicate adjectives usually agree with the noun they describe.

So:

• piasek = masculine singular
• suchy = dry, masculine singular
• ciepły = warm, masculine singular

That is why you get:

• piasek jest suchy i ciepły

Because piasek is a masculine singular noun.

The adjective forms must match the noun:

• masculine singular: suchy, ciepły
• feminine singular: sucha, ciepła
• neuter singular: suche, ciepłe
• plural personal/non-personal patterns vary

Since the subject is piasek, the masculine singular forms are required.

Polish does not have articles like English a/an and the.

That means Polish usually shows definiteness through:

• context
• word order
• demonstratives such as ten / ta / to

In this sentence, tej is not an article. It is a demonstrative meaning this.

So English needs something like this sandbox or the sandbox, but Polish does not use a separate word equivalent to the.

Yes. Polish word order is more flexible than English word order.

The sentence:

• W tej piaskownicy dobrze się bawi, bo piasek jest suchy i ciepły.

is natural, but you could also say:

• Dobrze się bawi w tej piaskownicy, bo piasek jest suchy i ciepły.

The difference is mainly emphasis:

• W tej piaskownicy... highlights the sandbox
• Dobrze się bawi... highlights having fun

The basic meaning stays the same.