Puer inani ventre dormire non potest.

Because potest means is able or can, and in Latin possum normally takes an infinitive.

So:

• dormire = to sleep
• potest = he is able / he can

Together, dormire potest means he can sleep, and dormire non potest means he cannot sleep.

If you used dormit, that would mean he sleeps / he is sleeping, which would make a different structure.

Puer is nominative singular, so it is the subject: the boy.

We know it is the subject because:

• puer is a nominative form
• potest is 3rd person singular
• the subject and verb match in number: the boy ... can/cannot

So puer is the one who cannot sleep.

Inani ventre is an ablative phrase meaning with an empty stomach/belly.

Literally:

• ventre = belly, stomach in the ablative singular
• inani = empty in the ablative singular, agreeing with ventre

This kind of ablative often expresses a circumstance or condition, and in English we often translate it with with:

• inani ventre = with an empty stomach
• more naturally in English: on an empty stomach

Because inani has to agree with ventre.

Here:

• ventre is ablative singular
• so the adjective describing it must also be ablative singular
• therefore we get inani

So the phrase works as:

• ventre = stomach/belly
• inani ventre = with an empty stomach

If it were a direct object in the accusative, then you might expect a different ending, but that is not what is happening here.

No. It is better understood simply as an ablative phrase expressing circumstance or condition.

An ablative absolute usually has:

• a noun or pronoun in the ablative
• plus a participle in the ablative

For example, something like duce veniente = with the leader coming.

But inani ventre has no participle. It just means with an empty stomach.

Yes, Latin word order is much freer than English word order.

This sentence puts:

• Puer first, introducing the topic: the boy
• inani ventre next, giving the circumstance
• dormire non potest at the end, giving the main action and negation

A different order could still make sense, such as:

• Puer non potest dormire
• Inani ventre puer dormire non potest
• Dormire puer inani ventre non potest

The endings tell you the grammar, so the order is often used for emphasis, rhythm, or style rather than basic sentence function.

Because non is negating the finite verb potest: cannot.

So:

• potest = he can
• non potest = he cannot

Since potest governs the infinitive dormire, the whole idea becomes he cannot sleep.

Latin often places non right before the word it most directly negates. Here that is potest.

Literally, venter is often belly or stomach.

In this sentence, English usually translates it more naturally as stomach, especially in the expression:

• with an empty stomach
• on an empty stomach

So the Latin is fairly concrete: empty belly/stomach, but the natural English meaning is the usual idiom about being hungry.

Potest is present tense: he can or he is able.

So the sentence describes a present situation or a general fact:

• The boy cannot sleep with an empty stomach
• or The boy is unable to sleep when his stomach is empty

The infinitive dormire itself does not carry tense in the same way here; the time is mainly determined by potest.

Yes. Very literally, it is:

• to sleep he is not able

But natural English says:

• he cannot sleep

That is a very common difference between Latin and English:

• Latin: possum + infinitive
• English: can + base verb

So dormire non potest is grammatically built as is not able to sleep, even though cannot sleep is the best normal translation.