Ni devas esti pretaj por la baldaŭ komenciĝonta kunveno.

Because it describes ni = we.

In Esperanto, adjectives agree with the nouns or pronouns they describe in both number and case. Since ni is plural, preta becomes pretaj.

• mi estas preta = I am ready
• li estas preta = he is ready
• ni estas pretaj = we are ready

So in this sentence, pretaj matches ni.

Because after devas you normally use an infinitive verb.

• devi = to have to, must
• esti = to be

So:

• Ni devas esti pretaj = We must be ready

You cannot normally say Ni devas pretaj, because pretaj is an adjective, not a verb. Esperanto needs esti here, just as English needs be in must be ready.

It means something like about to begin soon or soon-to-start.

The key part is komenciĝonta, which is a future active participle.

Breakdown:

• komenci = to begin, to start
• komenciĝi = to begin, to start, to get underway
• komenciĝonta = going to begin / about to begin

Then baldaŭ adds the idea of soon.

So:

• la baldaŭ komenciĝonta kunveno = the meeting that will begin soon

This is a compact participial way of saying something that English often expresses with a relative clause.

Because komenciĝi means to begin in the sense of to start happening, while komenci often means to start something.

Compare:

• Mi komencas la kunvenon = I start the meeting
• La kunveno komenciĝas = The meeting begins

So if you are describing the meeting, Esperanto naturally uses the intransitive form:

• komenciĝonta kunveno = a meeting that is going to begin

Without -iĝ-, komenconta kunveno would sound like a meeting that is going to start something, which is not the intended meaning.

-onta is the future active participle.

It describes someone or something that will do the action.

Examples:

• venonta gasto = a guest who will come
• forironta trajno = a train that is going to leave
• komenciĝonta kunveno = a meeting that is going to begin

So komenciĝonta means that will begin or about to begin.

Because the sentence refers to a specific meeting, not just any meeting.

• kunveno = a meeting
• la kunveno = the meeting

The participial phrase baldaŭ komenciĝonta describes that meeting, but the whole noun phrase is still definite:

• la baldaŭ komenciĝonta kunveno = the soon-starting meeting / the meeting that will begin soon

In Esperanto, la usually comes before the whole noun phrase.

Por here means for in the sense of in preparation for.

So:

• pretaj por la kunveno = ready for the meeting

This is very similar to English.

Other examples:

• Mi preparas min por la ekzameno = I prepare myself for the exam
• Ili estas pretaj por vojaĝi = They are ready to travel

In your sentence, por links pretaj with the thing you need to be ready for.

Yes. A very natural alternative is:

• Ni devas esti pretaj por la kunveno, kiu baldaŭ komenciĝos.

That means:

• We must be ready for the meeting, which will begin soon.

The participial version is more compact:

• la baldaŭ komenciĝonta kunveno

The clause version may feel easier for learners at first:

• la kunveno, kiu baldaŭ komenciĝos

Both are correct.

Here, baldaŭ modifies komenciĝonta, so it tells you when the meeting is going to begin.

• la baldaŭ komenciĝonta kunveno = the meeting that will soon begin

This placement is natural and clear.

You may also see similar wording in other structures, such as:

• la kunveno, kiu baldaŭ komenciĝos

The important idea is that baldaŭ belongs semantically with the beginning of the meeting, not with pretaj.

Yes.

Participles in Esperanto often behave like adjectives when they modify nouns.

So in:

• komenciĝonta kunveno

the word komenciĝonta describes kunveno just like a normal adjective would.

Because kunveno is singular and not accusative, the participle also has the plain adjective ending -a:

• komenciĝonta

If the noun were plural, the participle would also become plural:

• baldaŭ komenciĝontaj kunvenoj = meetings that will begin soon

The structure is:

• Ni = subject
• devas esti = verb phrase, must be
• pretaj = predicate adjective, describing ni
• por la baldaŭ komenciĝonta kunveno = prepositional phrase, for the soon-beginning meeting

So the sentence works like this:

• Ni
• devas esti
• pretaj
• por la baldaŭ komenciĝonta kunveno

In more natural English structure:

• We must be ready for the meeting that will begin soon.

Literally, yes, but that would sound awkward in normal English.

A more natural English rendering would be:

• the meeting that will begin soon
• the soon-to-begin meeting
• the upcoming meeting

This is a common issue when learning Esperanto participles: the Esperanto form is compact and adjective-like, but English often prefers a relative clause or a different expression.

Yes, it is grammatical and natural, especially in a somewhat formal or written style.

The participial phrase la baldaŭ komenciĝonta kunveno is perfectly normal Esperanto. That said, learners and casual speakers may sometimes prefer the more explicit version:

• Ni devas esti pretaj por la kunveno, kiu baldaŭ komenciĝos.

So the original sentence is good Esperanto; it is just a bit more compact and advanced in style.