Aevum imperii longum est.

It breaks down like this:

• aevum = the subject, nominative singular neuter
• imperii = genitive singular, meaning of the empire
• longum = an adjective meaning long, agreeing with aevum
• est = is

So the structure is essentially:

[Subject] + [dependent genitive] + [predicate adjective] + [verb]

That is, The duration/age of the empire is long.

Because it shows possession or relationship: the age/duration of the empire.

In Latin, the genitive often does the job that English does with of. So:

• imperium = empire
• imperii = of the empire

Here, imperii depends on aevum, telling us whose aevum it is.

Yes, imperii could also look like nominative plural or vocative plural in some contexts. But here, context makes genitive singular the natural reading.

Why?

Because aevum commonly takes a genitive to specify whose age, duration, or lifetime is being discussed. So in this sentence, aevum imperii is best understood as the age/duration of the empire, not the empires.

Because longum agrees with aevum, and aevum is neuter singular.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

So:

• masculine singular: longus
• feminine singular: longa
• neuter singular: longum

Since aevum is neuter singular, the adjective must be longum.

It describes aevum, not imperii.

You can tell because longum matches aevum in gender, number, and case:

• aevum = nominative singular neuter
• longum = nominative singular neuter

But imperii is genitive, so longum is not agreeing with it.

Because it is a predicate adjective.

In a sentence like X is long, Latin usually puts both the subject and the predicate adjective in the nominative:

• aevum = nominative subject
• longum = nominative predicate adjective

This can confuse English speakers because longum looks like an accusative form. But in neuter singular, the nominative and accusative often look the same.

So here longum is nominative, not accusative.

Because Latin has no definite article like English the, and no indefinite article like a/an either.

So aevum imperii could be understood as:

• the age of the empire
• an age of the empire
• sometimes just age of the empire

The exact English wording depends on context, not on a separate Latin article.

Latin word order is much freer than English word order because the endings show the grammatical relationships.

Putting est at the end is very normal in Latin, especially in straightforward prose. The sentence could be rearranged in other ways, such as:

• Aevum imperii est longum
• Imperii aevum longum est
• Longum est aevum imperii

These all express basically the same idea, though the emphasis may shift slightly.

Sometimes Latin does omit forms of to be, especially in poetry or very compressed style. But in a plain sentence like this, est is perfectly normal and expected.

So yes, omission is possible in some contexts, but Aevum imperii longum est is the standard full form.

Aevum can mean things like:

• age
• lifetime
• duration
• long period of time

So it does not just mean a person's numerical age. In this sentence it suggests the span or duration of the empire.

That is why English might translate it in slightly different ways depending on context.

In a classical-style pronunciation, roughly:

EYE-wum im-peh-REE-ee LONG-um est

A few notes:

• ae in aevum is a diphthong, roughly like eye
• g in longum is always hard
• ii in imperii is pronounced separately

A more careful classical pronunciation would be something like:

AI̯-wum im-peh-ree-ee LONG-um est

It is grammatically natural Latin.

A learner may notice that English often prefers something like The empire lasts a long time rather than The age of the empire is long, but Latin is very comfortable expressing an idea through a noun like aevum plus a predicate adjective like longum.

So the sentence is a normal kind of Latin construction, even if the most natural English translation might be phrased differently.