Praefurnium prope fornacem est.

Because prope takes the accusative case when it is used as a preposition meaning near or close to.

• Dictionary form: fornax = furnace, oven
• Accusative singular: fornacem

So prope fornacem means near the furnace.

Praefurnium is in the nominative singular, because it is the subject of the sentence.

The sentence is built like this:

• Praefurnium = the stokehole / furnace room / firebox area
• prope fornacem = near the furnace
• est = is

So praefurnium is the thing that is somewhere.

It is singular because the subject, praefurnium, is singular.

• praefurnium = one stokehole / one furnace room
• est = is

If the subject were plural, Latin would use a plural verb, such as sunt = are.

Latin usually has no definite article and no indefinite article.

So:

• praefurnium can mean the stokehole, a stokehole, or just stokehole
• fornacem can mean the furnace or a furnace

The correct English article depends on the context.

Latin word order is much more flexible than English word order because Latin uses case endings to show grammatical relationships.

English depends heavily on position:

• The stokehole is near the furnace

Latin can often move words around without changing the basic meaning:

• Praefurnium prope fornacem est
• Prope fornacem praefurnium est
• Est praefurnium prope fornacem

These may sound slightly different in emphasis, but the core meaning stays the same.

Often, yes, but it can also be used as an adverb.

In this sentence, it is clearly a preposition because it is followed by a noun:

• prope fornacem = near the furnace

As a preposition, prope normally takes the accusative.

Est is a verb, specifically the 3rd person singular present active indicative of sum, meaning to be.

So:

• sum = I am
• es = you are
• est = he/she/it is

Here it means is.

The ending -em shows that fornacem is accusative singular.

For this third-declension noun:

• nominative singular: fornax
• accusative singular: fornacem

A plural accusative form would look different.

Fornax is a third-declension noun.

That is why its forms change in a way that may look less familiar to beginners:

• nominative singular: fornax
• accusative singular: fornacem

This is different from first- or second-declension nouns, which often have more predictable endings for beginners.

Yes. In this sentence it clearly describes location:

• Praefurnium prope fornacem est = the stokehole is near the furnace

There is no idea of movement here.
The verb est simply states where something is.

A simple classroom-style pronunciation would be something like:

pry-FOOR-nee-um PRO-pe for-NA-kem est

More carefully by syllables:

• Prae-fur-ni-um
• pro-pe
• for-na-cem
• est

If your course uses restored classical pronunciation, c in fornacem is pronounced as a hard k sound.