Hoc saeculum longum est.

Because saeculum is a neuter singular noun, and hoc is the neuter singular nominative form of hic, haec, hoc.

Latin words that modify a noun usually have to agree with that noun in:

• gender
• number
• case

So here:

• saeculum = neuter, singular, nominative
• hoc = neuter, singular, nominative

That is why hoc saeculum is correct.

It is nominative singular.

In this sentence, saeculum is the subject, and the subject of a sentence with est is normally in the nominative.

So:

• saeculum = the thing being described
• longum = what is being said about it
• est = links them

Because saeculum is a second-declension neuter noun.

In the second declension:

• masculine nominative singular often ends in -us
• neuter nominative singular often ends in -um

So saeculum is nominative singular even though it ends in -um.

This is very common in Latin. For neuter second-declension nouns, -um is exactly what you expect in the nominative singular.

Because longum is an adjective agreeing with saeculum.

The adjective longus, longa, longum means long, and here it must match saeculum in:

• gender: neuter
• number: singular
• case: nominative

So:

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

Since saeculum is neuter singular nominative, the adjective must be longum.

In form, longum could be either nominative singular neuter or accusative singular neuter, because those two forms are identical in many neuter words.

But here it is understood as nominative, because it is a predicate adjective with est.

In a sentence like this, Latin uses:

• a nominative subject
• a nominative predicate adjective

So even though the form looks the same as an accusative, its job in the sentence is nominative.

Here it is an adjective, because it directly modifies the noun saeculum.

Compare:

• hoc saeculum = this age / this century → adjective use
• hoc longum est = this is long → pronoun use

So in your sentence, hoc is functioning adjectivally.

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

Placing est at the end is very normal Latin style. In fact, forms of esse often come late in the sentence.

So:

• Hoc saeculum longum est is perfectly natural.

Latin can often rearrange the words without changing the basic meaning, though the emphasis may change.

Yes, often they could.

For example, Latin could also have:

• Saeculum hoc longum est
• Longum hoc saeculum est

These do not all sound equally neutral, and some orders give different emphasis, but the core meaning can remain the same because the endings show the grammatical relationships.

Still, hoc saeculum longum est is a straightforward, readable order:

• hoc saeculum stays together as a noun phrase
• est comes at the end, which is common in Latin

Saeculum can mean several related things depending on context, such as:

• age
• generation
• century
• era

So the exact English word depends on the situation.

That is normal in Latin: one word may cover a range that English divides into several words.

Because Classical Latin has no articles.

English uses:

• a
• an
• the

Latin usually does not. So hoc saeculum does not need an extra word for the.

Whether English translates it as this age, this century, or something similar depends on context, not on an article in the Latin.

Yes. It is a noun phrase made of:

• hoc = demonstrative adjective
• saeculum = noun

Together they mean something like this age or this century.

A useful way to read the sentence is in chunks:

• hoc saeculum = the subject phrase
• longum = the description
• est = the linking verb

That can make the sentence easier to understand structurally.

Yes, normally they do.

In a sentence with esse such as this one, the adjective linked to the subject agrees with the subject in:

• gender
• number
• case

So because saeculum is neuter singular nominative, longum must also be neuter singular nominative.

This is an important difference from English, where adjectives do not change form:

• English: long stays long
• Latin: longus, longa, longum changes form to match the noun