La posedanto havas la potencon decidi.

It is built from:

• posedi = to possess / to own
• -ant- = the active participle idea, someone doing the action
• -o = noun ending

So posedanto literally means one who possesses, which in normal English is owner.

Because it is the direct object of havas.

The sentence says the owner has something: la potencon. In Esperanto, direct objects take -n.

So:

• La posedanto havas = The owner has
• la potencon = the power

Because la never changes.

In Esperanto, the definite article is always just la, whether the noun is singular, plural, nominative, or accusative. The case ending goes on the noun, and on any adjectives, but not on la.

So:

• la potenco = the power
• la potencon = the power as a direct object

Because Esperanto uses the infinitive to express to decide, just like English does.

Here decidi explains what kind of power is meant: the power to decide.

So:

• decido = a decision (noun)
• decidi = to decide (verb, infinitive)

The sentence needs decidi, not decido, because it is talking about the ability or authority to perform an action.

Because in Esperanto, the infinitive ending -i already carries that meaning.

English needs two words:

• to decide

Esperanto uses one word:

• decidi

So the to is already built into the form decidi.

Yes. That is normal Esperanto.

A noun can be followed by an infinitive when the infinitive explains the action connected with that noun. Here, la potencon decidi means the power to decide.

This is similar to other expressions such as:

• la rajto voĉdoni = the right to vote
• la ebleco elekti = the possibility to choose

So potencon decidi is a natural structure.

This is a very useful distinction.

• povo usually means ability / capability
• potenco usually means power / authority / control

So:

• La posedanto povas decidi = The owner can decide
  This stresses ability or possibility.

• La posedanto havas la potencon decidi = The owner has the power to decide
  This often suggests authority or the right to make the decision.

In many contexts both could work, but potenco sounds stronger and more about authority.

Because la marks something definite.

• la posedanto = the owner, a specific owner already known from context
• la potencon = the power, the specific power being talked about

If you removed la, the meaning could become less definite or more general, depending on context.

Yes, but it is not exactly the same.

• La posedanto povas decidi = The owner can decide
• La posedanto havas la potencon decidi = The owner has the power/authority to decide

The first version focuses more on ability or possibility.
The second version emphasizes authority, control, or official decision-making power.

This is the most neutral word order:

• La posedanto = subject
• havas = verb
• la potencon = object
• decidi = infinitive connected with potencon

Esperanto word order is somewhat flexible, but this version is the clearest and most natural in ordinary usage. The accusative -n on potencon helps show its role even if word order changes.