Στην παραλία η ανιψιά μου γεμίζει το κουβαδάκι με άμμο και το αδειάζει ξανά στο ίδιο σημείο.

Because στην is the usual contracted form of σε + την.

• σε = in / at / to
• την = the feminine singular article in the accusative

So:

• σε την παραλία → στην παραλία

This contraction is extremely common in Modern Greek:

• σε το → στο
• σε τη(ν) → στη(ν)

After σε (and therefore after στη(ν)), Greek normally uses the accusative case.

So παραλία here is in the accusative singular:

• nominative: η παραλία
• accusative: την παραλία

Even though English says at the beach or on the beach, Greek uses σε + accusative for this kind of location.

Greek often drops subject pronouns when they are not needed, because the verb ending already shows who the subject is.

• γεμίζω = I fill
• γεμίζεις = you fill
• γεμίζει = he/she/it fills

Since the sentence already has η ανιψιά μου as the subject, adding αυτή would be unnecessary unless you wanted extra emphasis.

So Greek naturally says:

• η ανιψιά μου γεμίζει...

not usually

• η ανιψιά μου αυτή γεμίζει...

In Modern Greek, possessive words like μου often come after the noun.

So:

• η ανιψιά μου = my niece

This is the most normal order in everyday Greek.

A few useful patterns:

• ο φίλος μου = my friend
• η αδερφή μου = my sister
• το σπίτι μου = my house

Greek also keeps the article:

• not just ανιψιά μου
• but usually η ανιψιά μου

Ανιψιά means niece.

Related words:

• ο ανιψιός = nephew
• η ανιψιά = niece

So the feminine ending -ιά here helps show that the person is female.

Yes. In Greek, the present tense can describe:

• something happening right now
• something that happens repeatedly
• a vivid scene being described

So this sentence could mean something like:

• she is filling the bucket and emptying it again
• or she does this repeatedly at the beach

Greek present tense often covers both meanings that English may express with either simple present or present progressive.

Κουβαδάκι is a diminutive form. It means a small bucket, often the kind a child uses at the beach.

The ending -άκι is very common in Greek and can suggest:

• small size
• affection
• something cute or child-related

So:

• κουβάς = bucket
• κουβαδάκι = little bucket / small bucket

In this sentence, κουβαδάκι sounds very natural because it fits a beach/play context.

Because with the verb γεμίζω, Greek commonly says what something is filled with by using με.

So:

• γεμίζω το κουβαδάκι με άμμο = fill the bucket with sand

This is very close to English here.

Also note that after με, the noun is in the accusative form. In this case:

• η άμμος = the sand
• άμμο = sand (accusative singular, without the article here)

That το is not the article. It is a direct object pronoun meaning it.

It refers back to το κουβαδάκι.

So the structure is:

• γεμίζει το κουβαδάκι = she fills the bucket
• και το αδειάζει = and empties it

Greek often uses these short object pronouns before the verb:

• τον βλέπω = I see him
• την ξέρω = I know her
• το θέλω = I want it

Here, το matches κουβαδάκι, which is neuter singular.

Because αδειάζω needs an object if you mean empty something.

If Greek only said:

• και αδειάζει

that could sound incomplete, or it might suggest becomes empty depending on context.

By saying:

• και το αδειάζει

Greek clearly means:

• she empties it = the bucket

So the pronoun helps keep the meaning precise and natural.

Ξανά means again.

So:

• το αδειάζει ξανά = she empties it again

Its placement after the verb is very natural. Greek adverbs are fairly flexible, but this position is common and easy to understand.

You may also see ξανά in other positions depending on emphasis, for example:

• ξανά το αδειάζει
• το ξανααδειάζει (with ξανα- attached in some contexts)

But in your sentence, το αδειάζει ξανά is perfectly normal.

Στο is another contraction:

• σε + το → στο

Then:

• ίδιο = same
• σημείο = spot / point / place

So:

• στο ίδιο σημείο = in / at the same spot

Everything is in the neuter singular accusative because it goes with το σημείο after σε:

• nominative: το ίδιο σημείο
• accusative: το ίδιο σημείο

For neuter nouns, nominative and accusative are often the same in form.

Yes, Greek word order is relatively flexible.

This sentence starts with Στην παραλία to set the scene first:

• At the beach, my niece...

That sounds very natural in Greek. But you could also say:

• Η ανιψιά μου γεμίζει το κουβαδάκι με άμμο στην παραλία...

The difference is mostly about focus and information flow, not basic meaning.

Starting with the place phrase helps the listener imagine the setting immediately.

Greek does not always use the article in the same places English does.

Here:

• το κουβαδάκι refers to a specific bucket in the scene
• το ίδιο σημείο refers to a specific spot

But με άμμο is more like naming the material being used, so Greek naturally leaves the article out.

Compare:

• γεμίζει το κουβαδάκι με άμμο = fills the bucket with sand
• παίζει με την άμμο = plays with the sand

Both are possible, but they express slightly different ways of viewing sand: as a material in general, or as a more specific thing in the scene.