Filia aquam in horto petit.

Because aquam is the direct object of the verb petit. In Latin, the direct object usually goes in the accusative case.

• aqua = nominative singular, water as a subject
• aquam = accusative singular, water as an object

So in this sentence, the daughter is doing the action, and water is what she is asking for or seeking, so Latin uses aquam.

Filia is in the nominative singular, which is the case normally used for the subject of a sentence.

So:

• filia = daughter as the one doing the action
• petit = she asks for / seeks

Even if the meaning were not given, the nominative form filia strongly suggests that the daughter is the subject.

Because in can take different cases depending on the meaning:

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here, in horto means in the garden, so it tells us where the action happens. That is why horto is in the ablative.

Compare:

• in horto = in the garden
• in hortum = into the garden

Horto is ablative singular of hortus, meaning garden.

It is ablative because it follows in in the sense of location: in the garden.

So the form works like this:

• hortus = nominative singular
• hortum = accusative singular
• horto = ablative singular

Petit is the 3rd person singular present active indicative of petere.

Depending on context, petere can mean things like:

• to seek
• to ask for
• to aim at
• to head for

In this sentence, with aquam, it most naturally means asks for water or seeks water.

So petit means she asks for / she seeks.

The ending -t tells us that the verb is 3rd person singular in the present tense.

So petit means:

• he seeks
• she seeks
• it seeks

Latin does not need a separate word for she here, because the verb ending already gives the person and number. Then filia makes it clear that the subject is the daughter, so in English we would use she.

Latin usually does not have articles like English the and a/an.

So filia can mean:

• a daughter
• the daughter

And aquam can mean:

• water
• sometimes more specifically the water, depending on context

Latin relies on context much more than English does for this. So the sentence itself does not force either the daughter or a daughter unless the larger context makes it clear.

Latin word order is more flexible than English word order, because the endings show the grammatical roles.

In this sentence:

• filia = subject
• aquam = object
• in horto = location
• petit = verb

A very common Latin pattern is to put the verb at the end, as happens here. But other orders are also possible, for example:

• Aquam filia in horto petit
• In horto filia aquam petit

These would still mean basically the same thing, though the emphasis might change slightly.

Here is the basic grammar of each word:

• filia — 1st declension noun
• aquam — from aqua, also 1st declension
• horto — from hortus, 2nd declension
• petit — from petere, a 3rd conjugation verb

That mix is very normal in Latin: nouns from different declensions and verbs from different conjugations can appear together in the same sentence.

Most naturally, it describes the situation of the action: the asking/seeking happens in the garden.

In other words, it tells us where the daughter is when she asks for the water.

Latin prepositional phrases like in horto often function this way, giving the setting or location of the action rather than acting like an adjective attached tightly to one noun.