Lucia usum huius vocabuli paulatim intellegit.

Usum is the accusative singular of usus, a fourth-declension noun meaning use.

It is in the accusative because it is the direct object of intellegit: Lucia understands what? — the use.

So the core structure is:

• Lucia = subject
• intellegit = verb
• usum = direct object

The dictionary form is usus, usūs (masculine), meaning use, practice, or experience, depending on context.

In this sentence, the important forms are:

• usus = nominative singular
• usum = accusative singular

A learner may expect a noun ending in -um to be second declension neuter, but here it is actually a fourth-declension masculine noun.

Because Latin uses the genitive case to express of this word.

Both words are genitive singular:

• huius = of this
• vocabuli = of word

Together, huius vocabuli modifies usum:

• usum huius vocabuli = the use of this word

This is very normal Latin structure: a noun followed by a genitive that depends on it.

Because the sentence needs the meaning of this word, not simply this word.

• hoc vocabulum would mean this word (nominative or accusative, depending on context)
• huius vocabuli means of this word (genitive)

So huius is used because the noun phrase is in the genitive.

Vocabuli is genitive singular of vocabulum, a second-declension neuter noun meaning word.

We know it is genitive singular because:

• vocabulum = nominative/accusative singular
• vocabuli = genitive singular
• vocabula = nominative/accusative plural

Here it must be genitive singular because it goes with huius and depends on usum:

• the use of this word

It describes usum, not Lucia.

More precisely, huius vocabuli is a genitive phrase attached to usum:

• usum = use
• huius vocabuli = of this word

So the meaning is the use of this word, not anything like Lucia of this word.

Paulatim is an adverb, meaning gradually, little by little, or step by step.

It modifies the verb intellegit:

• Lucia ... intellegit = Lucia understands
• Lucia ... paulatim intellegit = Lucia gradually understands

So it tells us how she comes to understand.

Yes, it could go elsewhere. Latin word order is relatively flexible.

This sentence has:

• Lucia usum huius vocabuli paulatim intellegit

But Latin could also say things like:

• Lucia paulatim usum huius vocabuli intellegit
• Usum huius vocabuli Lucia paulatim intellegit
• Lucia intellegit paulatim usum huius vocabuli

Some orders are more natural than others, but the basic grammar stays clear because the endings show the relationships.

Here, paulatim is placed near the verb, which is very natural for an adverb.

Intellegit is:

• third person singular
• present tense
• active voice
• indicative mood

from the verb intellegere, meaning to understand.

So intellegit means she understands or he understands.
Because the subject is Lucia, we know it means she understands.

Because the ending of the verb already tells you the subject is third person singular.

• intellegit = he/she/it understands

Latin often leaves out subject pronouns unless they are needed for emphasis or contrast.

So Lucia is enough to identify the subject, and there is no need to add ea.

Because Latin often places the topic or subject early in the sentence, especially in a simple statement.

So:

• Lucia comes first
• the object phrase follows
• the verb comes at the end

This is a very common and natural Latin pattern, though not a fixed rule.

Yes, it is a very natural construction.

Latin commonly uses:

• a verb of knowing or understanding
• plus a direct object
• plus a genitive phrase showing what that object belongs to or concerns

So usum huius vocabuli intellegit is a straightforward way to say she understands the use of this word.

In theory, huius by itself can mean of this person, of this thing, and so on, depending on context.

But in this sentence, it clearly goes with vocabuli:

• huius vocabuli = of this word

They form one noun phrase together.

Because the sentence is talking about this word, not these words.

Singular:

• huius vocabuli = of this word

Plural would be different:

• horum vocabulorum = of these words

So the Latin matches the singular idea exactly.

Yes. A more literal English rendering would be the use of this word or the usage of this word.

In good English, usage may sometimes sound a bit more natural in a grammar context, but Latin usus often covers both ideas. The sentence is simply saying that Lucia is gradually coming to understand how this word is used.

The structure is:

• Lucia — subject, nominative singular
• usum — direct object, accusative singular
• huius vocabuli — genitive phrase modifying usum
• paulatim — adverb
• intellegit — finite verb

So the sentence is built like this:

Lucia + understands + the use + of this word + gradually

or, in a more literal Latin order:

Lucia the use of this word gradually understands