Prope deversorium est basilica vetusta, cuius parietes e marmore facti sunt.

Prope means near or close to. When it is used as a preposition, it takes the accusative case.

So in prope deversorium, deversorium is accusative singular.

A detail that often confuses learners: deversorium is a neuter noun, and in many neuter nouns the nominative and accusative singular have the same form. So even though deversorium looks unchanged, its case here is accusative because prope requires it.

Latin word order is much freer than English word order. The sentence could have been arranged differently and still meant the same thing.

Here, Prope deversorium est basilica vetusta has the sense of Near the inn there is an old basilica. Putting est before basilica vetusta gives a natural there is / there exists feel.

So:

• est basilica vetusta = there is an old basilica
• basilica vetusta est would also be possible, but it would sound more like a simple statement about the basilica

In Latin, word order often reflects emphasis and style more than strict grammatical necessity.

Latin has no articles. It does not have separate words for a, an, or the.

So basilica vetusta can mean:

• an old basilica
• the old basilica

Which one is best depends on the context. In a sentence like this, English usually says an old basilica if it is being introduced for the first time.

In Latin, adjectives can come before or after the noun they describe. Both are common.

So:

• basilica vetusta
• vetusta basilica

both mean old basilica.

Here, vetusta simply agrees with basilica:

• basilica = feminine singular nominative
• vetusta = feminine singular nominative

The position after the noun is very normal Latin and does not change the basic meaning.

Cuius is the genitive singular of the relative pronoun qui, quae, quod.

Here it means whose or of which and introduces the relative clause:

cuius parietes e marmore facti sunt
= whose walls are made of marble

So the structure is:

• basilica vetusta = the antecedent
• cuius = whose, referring back to that basilica

A very important point: cuius is singular because it refers back to basilica, not to parietes.

Because the relative pronoun agrees with the noun it refers back to, not with the noun inside its own clause.

Here:

• antecedent: basilica = singular
• relative pronoun: cuius = singular

The word parietes is plural, but that does not control the number of cuius. The meaning is the basilica, whose walls...

In other words:

• whose = belonging to the basilica
• the walls are plural, but the owner is singular

Also, although basilica is feminine, the form cuius is the same for masculine, feminine, and neuter singular.

Parietes is nominative plural and is the subject of the relative clause.

In the clause:

cuius parietes e marmore facti sunt

the basic structure is:

• parietes = subject
• facti sunt = verb
• e marmore = phrase showing the material

So the walls are the thing being described as made of marble.

Marmore is ablative singular.

The preposition e or ex means out of or from, and it takes the ablative case. With materials, Latin often uses e/ex + ablative where English says made of.

So:

• e marmore = out of marble / of marble

This is a very common Latin way to express material.

E and ex are just two forms of the same preposition.

A common pattern is:

• e before consonants
• ex before vowels or h

Since marmore begins with m, e marmore is the expected form.

You may still sometimes see variation in Latin authors, but e marmore is perfectly standard.

Facti sunt is the perfect passive of facere.

It is made of two parts:

• facti = perfect passive participle
• sunt = they are

Together, literally, it means they have been made or they were made.

Because English often uses a simple present for material, we usually translate it more naturally as:

• are made of marble

The participle facti agrees with parietes:

• parietes = masculine plural
• facti = masculine plural

So the grammar is: the walls have been made from marble, which English normally expresses as the walls are made of marble.