Το αγόρι θέλει δύο παγάκια στο νερό του.

Because αγόρι is a neuter noun in Greek, even though it refers to a male person.

So:

• το αγόρι = the boy
• του αγοριού = of the boy / the boy’s

This is something English speakers often notice early: grammatical gender in Greek does not always match natural gender. A word can be grammatically neuter but still refer to a male person.

Θέλει is the 3rd person singular form of θέλω (to want).

Here the subject is το αγόρι = the boy, so Greek uses:

• θέλω = I want
• θέλεις = you want
• θέλει = he/she/it wants

So Το αγόρι θέλει... literally means The boy wants...

Because Greek does not need an indefinite article here.

In English, you might think of two ice cubes as having an understood some, but Greek simply says:

• δύο παγάκια = two ice cubes

The number itself already makes the phrase definite enough in meaning, so no extra article is needed.

Παγάκια is the plural of παγάκι, which means ice cube.

So:

• ένα παγάκι = one ice cube
• δύο παγάκια = two ice cubes

The word παγάκι is a very common everyday word for an ice cube. The ending -άκι is often a diminutive ending in Greek, but in this case παγάκι is just the normal everyday word learners should know.

Because after δύο (two), the noun appears in the normal plural form:

• δύο παγάκια

This is straightforward in Modern Greek:

• ένα παγάκι = one ice cube
• δύο παγάκια = two ice cubes
• τρία παγάκια = three ice cubes

Unlike some languages, Greek does not use a special counting form here. You just use the plural noun.

Στο is a contraction of:

• σε
  • το → στο

So:

• στο νερό = in the water / into the water

This contraction is extremely common in Greek:

• στο = σε + το
• στη = σε + τη(ν)
• στον = σε + τον

In this sentence, στο νερό του means in his water.

Because Greek usually puts the possessive pronoun after the noun.

So Greek says:

• το νερό του = literally the water his

But in natural English, that becomes:

• his water

This is very normal in Modern Greek:

• το βιβλίο μου = my book
• η φίλη σου = your friend
• το σπίτι τους = their house

So το νερό του is exactly the expected Greek structure.

Greek very often uses the definite article with possessed nouns.

So where English says:

• his water

Greek usually says:

• το νερό του

This is one of the biggest structural differences from English. The article is not optional in normal usage here; το νερό του is the natural phrase.

Here του means his.

In Modern Greek, του can function as a genitive/possessive form:

• το νερό του = his water

In this sentence, του refers back to το αγόρι, so the meaning is the boy wants two ice cubes in his water.

You will also see του in other roles, so context matters. But here it is clearly possessive.

Yes, Greek word order is more flexible than English, but this sentence is a very natural neutral order:

Το αγόρι θέλει δύο παγάκια στο νερό του.

That order presents the information clearly:

• subject: Το αγόρι
• verb: θέλει
• object: δύο παγάκια
• prepositional phrase: στο νερό του

A different order is possible for emphasis, but this version is the most straightforward for a learner.

It can feel like either, depending on context.

The preposition σε (inside στο) can cover meanings that English divides between in, into, at, and to, depending on the situation.

Here, with wanting two ice cubes in his water, English naturally says in his water, but the idea can also be understood as putting them into the water. Greek does not need to make that distinction as sharply here.