La kunveno komenciĝos je la oka kaj finiĝos je la naŭa.

Because -os is the Esperanto future-tense ending.

• komenciĝos = will begin
• finiĝos = will end

In Esperanto, verbs do not change for person:

• mi komenciĝos = I will begin
• ĝi komenciĝos = it will begin
• ili komenciĝos = they will begin

The future is always marked with -os.

-iĝ- often gives the idea of becoming, getting into a state, or happening by itself / intransitively.

So:

• komenci = to begin something, to start something
• komenciĝi = to begin, to start

and:

• fini = to finish something, to end something
• finiĝi = to finish, to end

In this sentence, the meeting itself is not actively starting another thing; rather, the meeting begins and ends. That is why komenciĝi and finiĝi are natural here.

Because komenci is usually transitive: it normally takes a direct object.

For example:

• Mi komencos la laboron. = I will start the work.

But if you want to say that something itself begins, Esperanto commonly uses komenciĝi:

• La kunveno komenciĝos. = The meeting will begin.

Using komencos without an object would sound incomplete or at least less standard to many learners and speakers.

Je is often used for clock times.

So:

• je la oka = at eight o’clock
• je la naŭa = at nine o’clock

With time expressions in Esperanto, je is a very common safe choice when no more specific preposition is needed.

Examples:

• je la tria = at three
• je la sepa vespere = at seven in the evening

Because telling the time in this pattern uses ordinal numbers, not cardinal numbers.

• ok = eight
• oka = eighth
• naŭ = nine
• naŭa = ninth

Esperanto often expresses clock time as literally at the eighth hour, at the ninth hour:

• je la oka = at eight o’clock
• je la naŭa = at nine o’clock

This is why the adjective ending -a appears.

Because this time expression is understood as something like at the eighth hour and at the ninth hour.

So la is part of the normal idiomatic structure:

• je la unua
• je la dua
• je la deka

Without la, it would sound unnatural in standard time-telling.

Because they are not direct objects here.

The structure is:

• je
  • time expression

After a preposition like je, you normally do not add -n unless there is some special reason involving motion or measure. Here there is no such reason.

So:

• je la oka is correct
• not je la okan

Kunveno comes from the idea of people coming together.

You can break it down like this:

• kun = with, together
• veni = to come
• kunveni = to gather, to assemble
• kunveno = a gathering, meeting

The noun ending -o shows that kunveno is a noun.

Kaj simply means and.

It links the two verb phrases:

• komenciĝos je la oka
• finiĝos je la naŭa

So the whole sentence says that the meeting will begin at one time and end at another.

It is usually better to keep it here.

• La kunveno komenciĝos je la oka kaj finiĝos je la naŭa.

Repeating je makes the sentence clear and balanced. In some contexts Esperanto can omit repeated words, but here including both prepositions is the most natural and easiest style for learners.

No, not really.

• La kunveno finiĝos = The meeting will end.
• La kunveno finos ... = The meeting will finish something.

So finos would usually need an object:

• La kunveno finos la diskuton. = The meeting will finish the discussion.

But if the meeting itself comes to an end, finiĝos is the normal choice.

A careful pronunciation is roughly:

ko-men-CEE-jos

And finiĝos is roughly:

fi-NEE-jos

A few details:

• c = ts
• ĝ = the j sound in judge
• stress is always on the second-to-last syllable

So:

• ko-men-ci-ĜOS
• fi-ni-ĜOS
• kun-VE-no
• NAŬ-a

Yes. Esperanto also allows fuller expressions such as:

• je la oka horo
• je la naŭa horo

But in ordinary usage, je la oka and je la naŭa are perfectly normal and more concise.

The structure is:

• La kunveno = subject
• komenciĝos = first verb
• je la oka = time expression
• kaj = and
• finiĝos = second verb
• je la naŭa = second time expression

So it follows a very straightforward pattern:

Subject + verb + time + and + verb + time

That is one reason Esperanto sentences like this are often easy to parse once you know the endings.