Ο μικρός κοιμάται μόνο με το αρκουδάκι του.

In Greek, an adjective can be used as a noun when it has an article.

So:

• μικρός = small, little
• ο μικρός = the little one / the little boy

This is very common in Greek. Depending on context, ο μικρός can mean:

• the little boy
• the younger one
• the son
• the little child

Here it refers to a male child.

μικρός is:

• masculine
• singular
• nominative

It matches ο, which is also masculine singular nominative.

So:

• ο μικρός = the little one (masculine subject)

It is in the nominative because it is the subject of the sentence.

κοιμάται is the 3rd person singular form of κοιμάμαι = to sleep.

So:

• κοιμάμαι = I sleep
• κοιμάσαι = you sleep
• κοιμάται = he/she sleeps

Even though it looks like a passive-type ending, κοιμάμαι is just the normal verb used for sleep in Modern Greek. It is not passive in meaning here.

So ο μικρός κοιμάται simply means the little boy sleeps.

This is an important difference.

• μόνος = alone, by oneself
• μόνο = only

In this sentence, μόνο means only.

So the idea is:

• κοιμάται μόνο με το αρκουδάκι του = he sleeps only with his teddy bear

That is different from:

• κοιμάται μόνος = he sleeps alone

English learners often confuse these because only and alone are related in meaning in some contexts, but Greek keeps them separate here.

It means only with his teddy bear.

The word μόνο is limiting the phrase με το αρκουδάκι του.

So the sense is:

• he will sleep only if he has his teddy bear
• he sleeps with nothing/nobody else, just his teddy bear

This is the most natural reading.

με means with.

It is a preposition, and in Modern Greek it is followed by the accusative case.

So:

• με το αρκουδάκι = with the teddy bear

That is why you see το αρκουδάκι and not a nominative form.

αρκουδάκι is a neuter singular noun, so it takes the neuter singular article το.

So:

• το αρκουδάκι = the teddy bear / the little bear

After με, the accusative singular neuter looks the same as the nominative, so the form stays το αρκουδάκι.

αρκουδάκι comes from αρκούδα = bear, with the diminutive ending -άκι.

So literally it means:

• little bear

But in normal usage, το αρκουδάκι often means:

• teddy bear
• a child’s stuffed bear

So in this sentence it most naturally means his teddy bear.

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

So:

• το αρκουδάκι του = his teddy bear

This is the normal Greek pattern.

Compare:

• το βιβλίο μου = my book
• η τσάντα σου = your bag
• το σπίτι τους = their house

So Greek usually says the teddy bear his, not his teddy bear.

It refers to the owner, not to the noun being owned.

Here του means his.

So:

• το αρκουδάκι του = his teddy bear

The noun αρκουδάκι is neuter, but that does not make the possessive neuter.
του tells you the owner is masculine (or sometimes just him in context).

If it were her teddy bear, you would say:

• το αρκουδάκι της

Yes, and that is exactly how the sentence is normally understood.

So:

• Ο μικρός ... το αρκουδάκι του = The little boy ... his teddy bear

Greek often uses this kind of possessive even when English might sometimes avoid repeating the owner.

Not exactly hidden, but Greek often uses adjectives by themselves when the noun is obvious from context.

So ο μικρός could stand for something like:

• ο μικρός γιος = the little son
• το μικρό παιδί would be neuter, so that is not what we have here
• simply the little boy / the little one

You do not need to imagine a specific omitted noun every time.
In many cases, ο μικρός works as a complete noun phrase by itself.

Yes. Greek word order is flexible.

The neutral order here is:

• Ο μικρός κοιμάται μόνο με το αρκουδάκι του.

But you could also hear things like:

• Μόνο με το αρκουδάκι του κοιμάται ο μικρός.
• Ο μικρός μόνο με το αρκουδάκι του κοιμάται.

These versions shift the emphasis, but the core meaning stays the same.

A simple breakdown is:

• Ο μικρός = the little boy / the little one (subject)
• κοιμάται = sleeps (verb)
• μόνο = only (adverb)
• με το αρκουδάκι του = with his teddy bear (prepositional phrase)

So the structure is:

subject + verb + adverb + prepositional phrase

Grammatically, it says:

• he sleeps only with his teddy bear

In real-life context, that often implies:

• he needs his teddy bear in order to sleep
• he won’t sleep unless he has it

So the exact nuance depends on context, but the sentence strongly suggests that the teddy bear is required for him to sleep comfortably.