Amphitheatrum hodie plenum est.

They are, but amphitheatrum is a second-declension neuter noun. In that pattern, the nominative singular and accusative singular have the same ending: -um.

So even though -um often makes learners think of the accusative, here amphitheatrum is nominative because it is the thing being described: the amphitheatre is full.

Amphitheatrum is neuter and belongs to the second declension.

Its basic form is amphitheatrum, amphitheatri. Because it is neuter:

• nominative singular = amphitheatrum
• accusative singular = amphitheatrum
• nominative plural = amphitheatra
• accusative plural = amphitheatra

That neuter pattern is very important in Latin.

Because plenum is agreeing with amphitheatrum.

Latin adjectives must agree with the noun they describe in:

• gender
• number
• case

Since amphitheatrum is neuter singular nominative, the adjective must also be neuter singular nominative, so we get plenum.

Compare:

• vir plenus est = the man is full
• puella plena est = the girl is full
• amphitheatrum plenum est = the amphitheatre is full

It is a predicate adjective.

That means it is linked to the subject by a form of to be, here est. So the structure is:

• amphitheatrum = subject
• est = is
• plenum = full

In other words, plenum is not a separate noun. It is describing the subject through the verb est.

Est is the third-person singular present indicative of esse, meaning to be.

So est means is.

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

Hodie is an adverb, meaning today.

Adverbs do not agree with nouns, so they do not change for gender, number, or case the way adjectives and nouns do. It simply tells you when the statement is true.

So:

• amphitheatrum = noun
• plenum = adjective
• est = verb
• hodie = adverb

Yes, that is very normal. Latin word order is much more flexible than English word order.

Putting the verb at or near the end is common, so Amphitheatrum hodie plenum est sounds natural.

Other orders are also possible, for example:

• Hodie amphitheatrum plenum est
• Amphitheatrum plenum est hodie

The basic meaning stays the same, though the emphasis can shift slightly.

Latin has no articles like English the or a/an.

So amphitheatrum can mean:

• the amphitheatre
• an amphitheatre
• simply amphitheatre

Which one sounds best in English depends on context. In this sentence, English usually says the amphitheatre is full today.

Not necessarily. Latin, like English, can leave that idea unstated if it is obvious from context.

So plenum est can simply mean is full.

If you wanted to specify what it is full of, Latin often uses plenus with a genitive or sometimes an ablative, depending on style and author. For example, you might see something like:

• plenum spectatorum = full of spectators

But in your sentence, that extra information is simply understood.

Yes. Since hodie is an adverb, it is fairly movable.

All of these are possible Latin word orders:

• Amphitheatrum hodie plenum est
• Hodie amphitheatrum plenum est
• Amphitheatrum plenum est hodie

The choice depends mostly on style and emphasis, not on a major change in meaning.