Retiarius adeo celeriter fugit ut secutor eum capere non possit.

It has two parts:

• Main clause: Retiarius adeo celeriter fugit
• Subordinate clause: ut secutor eum capere non possit

So the sentence means:

• first, the retiarius flees so quickly
• then, that the secutor cannot catch him

In the main clause, retiarius is the subject and fugit is the verb.
In the ut clause, secutor is the subject and possit is the finite verb.

This is a very common Latin pattern meaning so ... that.

• adeo = so, to such a degree
• ut = that

So:

• adeo celeriter = so quickly
• adeo celeriter fugit ut ... = he flees so quickly that ...

This strongly suggests a result clause, not a purpose clause. The first part sets up a degree or intensity, and the ut clause gives the result of that intensity.

Because ut introduces a result clause, and result clauses in Latin normally take the subjunctive.

So:

• potest = indicative, he can
• possit = present subjunctive, he may be able / can in a subordinate result clause

After adeo ... ut, the subjunctive is exactly what you expect.

Because this is a negative result clause.

In Latin:

• negative purpose clauses often use ne
• negative result clauses use ut
  • non

So:

• ut secutor eum capere non possit = that the secutor cannot catch him

A learner often expects ne, but here non is correct because the clause gives a result, not a purpose.

Because it depends on possit.

Latin uses a complementary infinitive after verbs like possum:

• potest capere = he can catch
• possit capere = he can / may be able to catch in the subjunctive

So capere is not a separate finite verb. It completes the meaning of possit.

Eum is accusative singular masculine.

Here it is the direct object of capere, so it means him:

• secutor eum capere non possit = the secutor cannot catch him

It refers back to retiarius.

A useful contrast:

• eum = him
• se = himself

If Latin had used se here, it would normally refer to the subject of its own clause, secutor, which would give the wrong meaning: the secutor cannot catch himself.

Formally, it can be either.

From fugio, fugere, fugi:

• present: fugit = he flees / is fleeing
• perfect: fugit = he fled / has fled

So the form is ambiguous by itself.

In this sentence, the context points to the present meaning:

• The retiarius flees so quickly that the secutor cannot catch him

That fits the ongoing, general sense of the sentence much better than a perfect.

Celeriter is an adverb, meaning quickly.

It modifies the verb fugit:

• fugit = he flees
• celeriter fugit = he flees quickly

And with adeo:

• adeo celeriter fugit = he flees so quickly

Since it is an adverb, it does not have case, number, or gender.

Both are nominative singular nouns, because each is the subject of its own clause.

• retiarius = subject of fugit
• secutor = subject of possit

They are also special cultural words: both are types of Roman gladiator.

So the sentence is not about random people named by ordinary job titles; it is specifically describing a gladiator scene.

Because Latin word order is much more flexible than English word order. Latin relies heavily on endings, not just position, to show what each word is doing.

This sentence puts:

• adeo celeriter before fugit for emphasis on the manner
• the ut clause after the main clause, which is very natural
• non right before possit, where it clearly negates that verb

English needs a stricter order, but Latin can move words around more freely without changing the basic meaning.

Yes. Latin often uses tam ... ut as well as adeo ... ut.

So a sentence like:

• Retiarius tam celeriter fugit ut secutor eum capere non possit

would mean essentially the same thing.

In this sentence, adeo simply gives the same so ... that idea in a slightly different way.