Circus hodie plenus est.

Latin has no articles. That means it has no separate words for the or a/an.

So circus can mean:

• the circus
• a circus

Which one is meant depends on context. In a simple sentence like this, English often uses the: The circus is full today.

Circus is nominative singular. It is the subject of the sentence: the thing being described as full.

It comes from the second-declension noun circus, circī. In the nominative singular, a masculine second-declension noun often ends in -us, so circus is the basic subject form.

Because plenus is an adjective, and Latin adjectives must agree with the noun they describe in:

• gender
• number
• case

Since circus is masculine singular nominative, plenus must also be masculine singular nominative.

So:

• circus = masculine singular nominative
• plenus = masculine singular nominative

They match.

Est means is. It is the third-person singular present form of esse, to be.

So the structure is:

• Circus = the subject
• plenus = the description
• est = is

Literally: The circus today full is, which in normal English becomes The circus is full today.

Latin often puts the verb near the end, but it does not always have to.

So Circus hodie plenus est is very normal Latin word order. But other orders are also possible, for example:

• Hodie circus plenus est
• Circus plenus est hodie

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

Latin word order is more flexible than English because Latin uses endings to show how words function.

Hodie is an adverb. It means today.

It modifies the whole statement, telling you when the circus is full.

Because it is an adverb, it does not change its ending to agree with any noun.

Yes. Plenus literally means full, but in natural English, if we are talking about a place full of people, we often say crowded.

So depending on context, a good translation could be:

• The circus is full today.
• The circus is crowded today.

Both can be reasonable.

In classical Latin, circus usually means a circular or oval arena, especially a racecourse like the Circus Maximus.

So to a Roman, circus would more naturally suggest a chariot-racing venue than a modern traveling circus.

In beginner textbook Latin, though, it is sometimes translated simply as circus because that is the closest English-looking word.

Sometimes, yes. Latin can occasionally omit forms of esse (to be), especially in certain styles or contexts.

But in a basic sentence like this, including est is completely normal and often clearer for learners:

• Circus hodie plenus est = standard full sentence

Without est, the meaning might still be understood in some contexts, but it would be less straightforward for a beginner.

The adjective would have to change to agree with the noun.

For example:

• schola hodie plena est = the school is full today
  (schola is feminine singular, so plenus becomes plena)

• circī hodie plenī sunt = the circuses are full today
  (circī is masculine plural, so plenus becomes plenī, and est becomes sunt)

This is a good example of how Latin agreement works.