Το παιδί κλωτσάει τη μπάλα στο πάρκο.

Because παιδί is a neuter noun in Greek. Greek nouns have grammatical gender: masculine, feminine, or neuter.

So:

• ο = masculine the
• η = feminine the
• το = neuter the

Even though παιδί refers to a person, its grammatical gender is still neuter. That is normal in Greek. Grammatical gender does not always match biological sex.

There are three main noun phrases here:

• το παιδί = nominative, because it is the subject
• τη μπάλα = accusative, because it is the direct object
• στο πάρκο = also accusative, because the preposition σε takes the accusative in Modern Greek

So Greek is marking grammatical roles partly through articles and partly through context.

Because η μπάλα is the subject form and τη μπάλα is the object form.

For this feminine noun:

• η μπάλα = nominative = subject
• τη μπάλα / την μπάλα = accusative = object

Since the ball is what is being kicked, Greek uses the accusative article.

Both forms exist in Modern Greek.

• την μπάλα is the fuller form
• τη μπάλα is a common shortened form, especially in everyday speech and informal writing

You will often hear and see both. In more careful or formal writing, many speakers prefer την before certain sounds, and μπ is one of those cases, so την μπάλα is also very common.

So the short answer is: τη is a normal reduced form of την.

κλωτσάει is the 3rd person singular present tense form of κλωτσάω.

That means it matches:

• το παιδί = he/she/it
• κλωτσάει = kicks / is kicking

In Modern Greek, the present tense can often cover both:

• a general action: kicks
• an action happening now: is kicking

Context tells you which one is meant.

Yes. In everyday Modern Greek, many verbs in -άω have two common present forms.

So you may hear:

• κλωτσάει
• κλωτσά

Both are natural. Κλωτσάει may sound a bit fuller, while κλωτσά is very common in speech.

Because στο is a contraction of:

• σε = in, at, to
• το = the

So:

• σε το πάρκο becomes στο πάρκο

This contraction is extremely common in Greek. Similar forms include:

• στον = σε + τον
• στη(ν) = σε + τη(ν)
• στα = σε + τα

Because many Greek neuter nouns look the same in the nominative and accusative singular.

So:

• το πάρκο = the park
• στο πάρκο = in the park

The noun itself does not change here. What changes is the structure around it, especially the article/preposition combination στο.

This is very common with neuter nouns like:

• το βιβλίο
• το σπίτι
• το παιδί
• το πάρκο

Because in Modern Greek, the letter β is pronounced like v, not like English b.

So Greek usually writes the b sound with μπ.

That means:

• β = v
• μπ = usually b at the beginning of a word

So μπάλα is pronounced roughly BA-la, not VA-la.

Greek word order is more flexible than English word order.

The sentence as given is a very neutral order:

• subject + verb + object + place

But Greek can move parts around for emphasis or style, for example:

• Στο πάρκο το παιδί κλωτσάει τη μπάλα = emphasis on in the park
• Τη μπάλα κλωτσάει το παιδί = emphasis on the ball

So the original sentence is the most straightforward version, but it is not the only possible one.

The accent mark shows which syllable is stressed.

So:

• παιδί → peh-THEE
• κλωτσάει → klo-TSA-ee
• μπάλα → BA-la
• πάρκο → PAR-ko

In Modern Greek, stress is very important, and the written accent helps you place it correctly. Usually, each word has one stressed syllable.