Kiam ni pretos, ni komencos la kunvenon.

Here kiam means when and introduces a time clause: when we are/will be ready.

In Esperanto, kiam can be used both:

• as a question word: Kiam vi venos? = When will you come?
• as a connecting word in a sentence: Kiam ni pretos, ni komencos... = When we are ready, we will start...

So in this sentence, it is not asking a question. It is linking the two parts of the sentence.

The ending -os marks the future tense in Esperanto.

• pretos = will be ready
• komencos = will start

A native English speaker may expect something like When we are ready, we will start..., with a present-tense form after when. English often does that.

Esperanto usually uses the tense that matches the real time of the action more directly. Since both actions are in the future, both verbs can naturally be in -os.

That is a very natural question.

Pretos is the future form of preti, which means to be ready.

So:

• ni pretas = we are ready
• ni pretos = we will be ready

You could also say:

• Kiam ni estos pretaj, ni komencos la kunvenon.

That uses esti + the adjective preta:

• estos = will be
• pretaj = ready (plural, because ni = we)

Both versions are understandable and natural. The sentence you were given uses the shorter verb form pretos.

Because pretos is a verb, not an adjective.

Compare:

• Ni pretos = We will be ready
• Ni estos pretaj = We will be ready

In the second version, pretaj is an adjective, so it takes the plural ending -j to agree with ni.

In the original sentence, pretos already functions as the verb, so there is no adjective ending there.

Because each clause normally has its own subject.

The sentence has two clauses:

• Kiam ni pretos
• ni komencos la kunvenon

Even though both subjects are we, Esperanto normally states the subject in each clause. Repeating ni makes the structure clear and natural.

The -n ending marks the direct object in Esperanto.

Here, la kunvenon is the thing being started:

• komenci ion = to start something

So:

• ni komencos la kunvenon = we will start the meeting

Without -n, kunveno would be the subject form, not the direct object form.

La is the definite article, meaning the.

So:

• la kunvenon = the meeting
• kunvenon = a meeting or just meeting in a less specific sense

Using la suggests that the speaker and listener already know which meeting is meant.

Yes, it is normal to use a comma here.

Kiam ni pretos is a subordinate clause, and it comes before the main clause:

• Kiam ni pretos, ni komencos la kunvenon.

The comma helps separate those two parts clearly. This is standard and recommended.

Yes. You can also say:

Ni komencos la kunvenon kiam ni pretos.

That means the same thing: We will start the meeting when we are ready.

The version with Kiam... first puts the time condition first for emphasis or flow. The version with the main clause first may sound a little more straightforward. Both are correct.

Kunveno means meeting, gathering, or session, depending on context.

It comes from the idea of people coming together:

• kun- = together
• the root relates to coming/meeting
• -o makes it a noun

So la kunvenon here is simply the meeting.