Puella lac calidum bibere non vult, quia os eius dolet.

Vult is the third-person singular present active indicative of volo, velle, which means to want.

So puella ... vult means the girl wants ... or, with non, the girl does not want ....

It is irregular, so it does not follow the most basic first-conjugation pattern that learners often expect.

After volo and other verbs of wanting, being able, beginning, and so on, Latin commonly uses an infinitive to express the action.

So:

• vult bibere = wants to drink
• not wants drinks

Here bibere is the present active infinitive of bibo, bibere.

Puella is nominative singular. It is the subject of vult.

In other words, the girl is the one who does not want to do something.

A learner often identifies the subject in Latin by:

• the nominative case
• and the verb ending

Here the sentence opens with puella, which makes the subject especially clear.

Lac is accusative singular here, because it is the direct object of bibere. It is the thing being drunk.

So:

• puella = subject
• lac = direct object

A tricky point is that lac is a neuter third-declension noun, and in neuter nouns the nominative and accusative are often the same in form. So you must tell its case from its job in the sentence, not just from its ending.

Because the adjective must agree with the noun it modifies in gender, number, and case.

Here:

• lac is neuter
• singular
• accusative

So the adjective must also be:

• neuter
• singular
• accusative

That gives calidum.

So lac calidum means warm milk or hot milk, with the adjective matching lac grammatically.

Because non is negating the verb vult.

So the main idea is:

• vult = wants
• non vult = does not want

Latin word order is flexible, but placing non right before the verb it negates is very common and easy to understand.

So bibere non vult naturally means does not want to drink.

Quia means because. It introduces a clause giving the reason.

So the sentence has:

• a main clause: Puella lac calidum bibere non vult
• a subordinate clause of reason: quia os eius dolet

That second clause explains why the girl does not want to drink the warm milk.

Because os is the subject of dolet.

Latin is saying, very literally:

• her mouth hurts
• or her mouth is in pain

So os is not the object of the verb. It is the thing that is hurting.

This is similar to English in sentences like my head hurts. The body part itself is the grammatical subject.

This is a very common question.

Eius means his, her, its, of him, of her, of it.
Suus, sua, suum is the reflexive possessive, used when the possessor is the subject of its own clause.

In quia os eius dolet, the subject of the clause is os. So a reflexive possessive like suum would point back to os, not to puella.

But the possessor we want is the girl, not the mouth itself. So Latin uses eius.

That is why os eius correctly means her mouth.

Eius is the genitive singular of is, ea, id.

Its form is the same for:

• masculine singular
• feminine singular
• neuter singular

So by itself, eius can mean:

• his
• her
• its
• of him
• of her
• of it

Context tells you which one is meant. Here, since the sentence is about puella, it means her.

Latin word order is much more flexible than English word order because the endings usually show how words function.

So this sentence could be rearranged in other ways and still mean basically the same thing, for example:

• Puella non vult lac calidum bibere, quia os eius dolet.
• Lac calidum puella bibere non vult, quia os eius dolet.

The version you were given is perfectly natural. Its order keeps the ideas clear:

• Puella sets up the subject
• lac calidum bibere gives the action she does not want to do
• non vult finishes the main clause
• quia os eius dolet gives the reason

Different word orders can add different emphasis, but the cases and verb forms keep the grammar understandable.