Ocrea crus Thraecis tegit.

Because Thraecis is in the genitive singular, which often shows possession or close relationship.

Here, crus Thraecis means the leg of a Thracian.

The dictionary form is:

• Thraex = a Thracian
• Thraecis = of a Thracian

So this is a third-declension noun, not a first- or second-declension one.

Because ocrea is in the nominative singular, which is the normal case for the subject.

Also, the verb tegit is singular, so it needs a singular subject.
That matches ocrea:

• ocrea = a greave
• tegit = covers / protects

So ocrea is the thing doing the action.

Because crus is a neuter noun of the third declension, and for many neuter nouns, the nominative and accusative singular are the same.

So:

• nominative singular: crus
• accusative singular: crus

In this sentence, crus is the direct object because it is the thing being covered or protected by the greave.

It is accusative singular here.

Even though the form is identical to the nominative, its job in the sentence shows that it is accusative:

• ocrea = subject
• tegit = verb
• crus = the thing being covered

So crus is the direct object of tegit.

Latin normally has no articles.

So ocrea can mean:

• a greave
• the greave

and crus Thraecis can mean:

• the leg of a Thracian
• a Thracian’s leg

The exact English wording depends on context.

Tegit comes from the verb tego, tegere, meaning to cover or to protect.

Tegit is:

• third person singular
• present tense
• active voice

So it means he/she/it covers or he/she/it protects.
Since the subject is ocrea, the natural translation is the greave covers/protects.

Because Latin word order is much more flexible than English word order.

Putting the verb at the end is very common in Latin.
So:

• Ocrea crus Thraecis tegit

is perfectly normal Latin word order.

English usually prefers subject-verb-object order, but Latin often saves the verb for last.

Yes. Latin endings show the grammar, so the order can change without changing the basic meaning.

For example, these would still mean roughly the same thing:

• Ocrea crus Thraecis tegit
• Crus Thraecis ocrea tegit
• Thraecis crus ocrea tegit

The endings tell you which word is subject, object, and genitive.
However, different orders can change the emphasis.

Because the noun is not a regular second-declension noun.

Its dictionary form is:

• Thraex, Thraecis

That means it belongs to the third declension.
So the genitive singular is Thraecis, not Thraecii.

This is a very common thing to check in Latin: the genitive form in the dictionary tells you how the noun declines.

Here it is singular: of a Thracian.

That is because it is the genitive singular of Thraex.

A learner might wonder whether it could be some kind of plural form, but in this sentence the meaning and vocabulary point to the singular genitive.

Ocrea is a first-declension feminine noun meaning greave, a piece of armor for the leg.

Its basic pattern is like many first-declension nouns:

• nominative singular: ocrea
• genitive singular: ocreae

In this sentence, it is nominative singular, so it is the subject.

It can mean either, depending on context.

The basic idea of tego is cover.
From that, protect is a natural extension.

So in this sentence, either of these works:

• A greave covers the leg of a Thracian
• A greave protects a Thracian’s leg

If the meaning has already been given to you, both are grammatically supported by the Latin.

Because Latin often places a genitive after the noun it modifies.

So:

• crus Thraecis = the leg of a Thracian

But Latin could also put the genitive before the noun:

• Thraecis crus

Both are possible. The choice is usually about style or emphasis, not basic grammar.

It breaks down like this:

• Ocrea = subject
• crus = direct object
• Thraecis = genitive modifying crus
• tegit = verb

So the sentence literally means:

• A greave the leg of a Thracian covers

And in normal English:

• A greave covers/protects the leg of a Thracian.