Specus sub aggere longus est.

Specus is nominative singular, because it is the subject of the sentence.

In Specus sub aggere longus est, the basic structure is:

• specus = the thing being talked about
• longus est = is long

So specus is the noun that is long.

Because longus is an adjective agreeing with specus.

Latin adjectives must match the noun they describe in:

• gender
• number
• case

Here, specus is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives longus.

No. Even though specus ends in -us, it is actually a fourth-declension noun.

That is something English-speaking learners often have to get used to: the ending -us does not always mean second declension.

For specus, the forms include things like:

• nominative singular: specus
• genitive singular: specūs
• accusative singular: specum
• ablative singular: specū

So its declension pattern is different from a second-declension noun like servus.

Aggere is ablative singular.

It is ablative because it follows sub in a sense of location:

• sub
  • ablative = under, beneath, in a place under something
• sub
  • accusative = movement to a place under something

So here sub aggere means under the embankment / beneath the mound, describing where the specus is.

This is a very common question.

With sub:

• ablative usually shows position: being under something
• accusative usually shows motion toward a place under something

So:

• sub aggere = under the embankment / beneath the embankment
  (location)
• sub aggerem = to under the embankment / under the embankment
  (movement toward that position)

In this sentence, there is no idea of movement, so the ablative aggere is used.

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

In English, we usually need something like:

• The cave is long under the embankment

But in Latin, the verb often comes near the end, especially in simple statements. So Specus sub aggere longus est is a very normal Latin order.

The endings already show how the words relate, so Latin does not depend as heavily on position as English does.

Yes. Latin allows a good deal of rearrangement.

For example, these could still express roughly the same basic meaning:

• Specus longus sub aggere est
• Sub aggere specus longus est
• Longus est specus sub aggere

The exact emphasis may shift a little depending on the order, but the grammar stays clear because of the endings.

No. Classical Latin does not have definite or indefinite articles like English the and a/an.

So specus can mean:

• a cave
• the cave
• a tunnel
• the tunnel

And aggere can mean:

• an embankment
• the embankment

The context tells you which English article makes most sense.

It tells you the location of the specus.

So the sentence is saying that the specus is under the embankment, and that it is long.

In other words, sub aggere goes naturally with the whole idea of the cave’s position, not just with longus by itself.

Because Latin does not need adjectives to stand right beside their nouns as strictly as English does.

English usually prefers:

• the long cave

Latin can do that too, but it can also separate adjective and noun because their endings show that they belong together.

Here:

• specus = nominative singular masculine
• longus = nominative singular masculine

That matching tells you that longus describes specus, even though sub aggere comes between them.