Το μικρό αγόρι πήρε ένα κόκκινο κουβαδάκι, ενώ η αδερφή του μάζευε κοχύλια.

Because Greek grammatical gender does not always match biological sex. Αγόρι is a neuter noun, so it takes neuter forms like το μικρό αγόρι.

This is something English speakers often have to get used to:

• grammatical gender = a property of the noun
• natural gender = whether the person is male or female

So even though the referent is male, the noun αγόρι is still grammatically neuter.

They match the nouns they describe.

Both αγόρι and κουβαδάκι are:

• neuter
• singular

So the article and adjectives must also be in the neuter singular form:

• το μικρό αγόρι
• ένα κόκκινο κουβαδάκι

That is called agreement. In Greek, articles and adjectives agree with the noun in:

• gender
• number
• case

Because κουβαδάκι is neuter.

The indefinite article changes by gender:

• masculine: ένας / έναν
• feminine: μια or μία
• neuter: ένα

So:

• ένα κουβαδάκι = correct, because κουβαδάκι is neuter
• έναν κουβά = would be possible with the masculine noun κουβάς

Also, in neuter singular, the nominative and accusative forms are usually the same, so ένα works here as the direct object too.

Κουβαδάκι is a diminutive form, meaning something like:

• little bucket
• small pail
• often a child’s beach bucket

The ending -άκι is a very common Greek diminutive suffix. It can show:

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

So κουβαδάκι comes from κουβάς = bucket, but sounds smaller and more natural in a beach/childhood context.

Yes, both are past, but they show different kinds of past action.

• πήρε = aorist
  This usually shows a single completed action: he took it.

• μάζευε = imperfect
  This usually shows an ongoing, repeated, or background action: she was collecting.

So the sentence paints this picture:

• one action happened at a point in time: πήρε
• another action was already in progress: μάζευε

That is why the combination feels natural in English as:

• The little boy took... while his sister was collecting shells.

Here ενώ means while.

It connects two actions happening at the same time:

• the boy took the bucket
• his sister was collecting shells

In other contexts, ενώ can also mean whereas or while on the other hand, introducing contrast. But in this sentence, the main idea is simultaneous action.

In Greek, short possessive words like μου, σου, του, της, μας, σας, τους usually come after the noun.

So:

• η αδερφή του = his sister

This του is the weak genitive pronoun.

Important point: του can mean:

• his
• her
• sometimes its

The exact meaning comes from context. In this sentence, it most naturally means his, referring to the boy.

Greek often leaves subject pronouns out when they are not needed. This is called a pro-drop language.

The verb form already shows the person and number, and in this sentence the subjects are named explicitly:

• Το μικρό αγόρι πήρε...
• η αδερφή του μάζευε...

So adding αυτός or αυτή would usually be unnecessary unless you wanted extra emphasis.

Because κοχύλια here is being used in an indefinite sense: shells, not the shells.

Greek often omits the article with an indefinite plural object, especially when you mean:

• some shells
• shells in general

So:

• μάζευε κοχύλια = she was collecting shells
• μάζευε τα κοχύλια = she was collecting the shells, meaning specific ones already known in the context

Yes, αδερφή is perfectly normal in Modern Greek.

Both forms exist:

• αδερφή = very common in everyday speech and writing
• αδελφή = more formal or more conservative spelling/style

The same is true with:

• αδερφός
• αδελφός

A learner will hear αδερφή very often in real life.

Yes. Greek word order is more flexible than English because the endings carry a lot of grammatical information.

This sentence uses a very natural, neutral order:

• Το μικρό αγόρι πήρε ένα κόκκινο κουβαδάκι, ενώ η αδερφή του μάζευε κοχύλια.

But Greek could change the order for emphasis, for example:

• Ενώ η αδερφή του μάζευε κοχύλια, το μικρό αγόρι πήρε ένα κόκκινο κουβαδάκι.

That still means basically the same thing, but the focus shifts slightly. The version you were given sounds like normal storytelling prose.