Rex regni potis est.

Here is the basic parsing:

• rex: nominative singular, the subject, from rex, regis
• regni: genitive singular, from regnum, regni
• potis: an adjective used predicatively
• est: he/she/it is, 3rd person singular present of sum

So the skeleton of the sentence is:

• subject: rex
• predicate: potis est
• dependent genitive: regni

Because potis can take a genitive to show the thing or sphere over which someone has power or control.

So regni does not mean simple possession in the everyday English sense of of the kingdom. It is a genitive used with potis: master of / having power over the kingdom.

Many grammars would call this a dependent genitive or an objective genitive.

Here potis means something like powerful over, master of, or having control over.

It is not the most common everyday adjective for powerful, but in this construction it has a clear idiomatic sense with the genitive. So regni potis means having power over the kingdom or master of the kingdom.

It is an adjective, not a verb.

In this sentence it works as a predicate adjective with est, just as in English you can say the king is strong. Here the pattern is the king is potis—that is, the king is master / has power.

So the finite verb is est, and potis describes the subject rex.

Because potis by itself is not a finite verb. Latin needs est to say is.

So:

• potis = the descriptive idea
• est = the actual verb

Together they make the full predicate potis est.

Latin can sometimes leave out est when it is understood, especially in certain styles, but here it is stated explicitly.

Yes—historically, very much so.

Potest is historically connected with potis est. In fact, the verb possum comes from the old combination potis + sum.

But in this sentence you should analyze potis est as a separate adjective plus est, not as the ordinary verb potest. That matters because potis here takes regni in the genitive, which is not how possum normally works.

Because Latin has no articles.

So rex can mean:

• the king
• a king

Which one is meant depends on context. In a short example like this, English usually supplies whichever article makes best sense.

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

This sentence puts:

• rex first as the topic or subject
• regni right before potis, which helps show that the genitive belongs with potis
• est last, which is very common in Latin

Other orders are possible, such as Rex potis est regni, though the given order is neat and natural. Latin word order often reflects emphasis and style more than strict grammatical necessity.

Often, yes. Potens is generally the more familiar adjective for powerful.

But potis is a real Latin word and has its own flavor, especially in constructions like this one. It can sound more literary or more specialized, especially when used with the genitive to mean master of or having power over.

So a learner should recognize potis as perfectly valid, even if potens may feel more familiar from textbooks.

It depends on potis, not directly on rex.

That is an important point. The idea is not simply the king of the kingdom. If Latin meant that, you would be looking at a different construction.

Here the sense is:

• rex = the subject
• potis = the adjective meaning master / in control
• regni = the thing over which that control extends

So regni belongs with potis.