Ĉiu el ni devas esti preta por la kunveno.

Ĉiu means each one / every individual one, so it is grammatically singular.

In Ĉiu el ni devas esti preta, the idea is each of us, one by one, must be ready. Because ĉiu is singular, the rest of the sentence also stays singular:

• ĉiu = each
• devas = must
• preta = ready (singular)

If you used ĉiuj, that would mean all of us as a plural group. For example:

• Ni ĉiuj devas esti pretaj. = We all must be ready.

Both are possible, but they stress things a little differently:

• Ĉiu el ni... = each individual person
• Ni ĉiuj... = all of us together

El means out of, from among.

So ĉiu el ni literally means each out of us, which is how Esperanto expresses each of us.

This is a very common pattern:

• unu el ili = one of them
• kelkaj el la libroj = some of the books
• neniu el ni = none of us

English uses of, but Esperanto often uses el for this kind of from among a group meaning.

After a preposition like el, Esperanto normally uses the basic pronoun form, not the accusative -n form.

So:

• el mi
• el ni
• por ŝi
• kun ili

The -n ending is mainly for the direct object, and sometimes for motion toward something. Here, ni is simply part of the prepositional phrase el ni, so there is no -n.

Because it agrees with ĉiu, which is singular.

Even though ni refers to several people, the sentence is built around ĉiu = each one. Esperanto makes the adjective match that singular idea:

• Ĉiu el ni estas preta.
• Ĉiu studento estas preta.

Compare that with a plural subject:

• Ni estas pretaj.
• La studentoj estas pretaj.

So in your sentence, preta is singular because ĉiu is singular.

In Esperanto, if you want to say someone must be something, you need the infinitive esti.

So:

• devas esti preta = must be ready
• povas esti utila = can be useful
• volas esti helpema = wants to be helpful

After a modal verb like devas, the next verb stays in the infinitive form:

• devas iri = must go
• devas labori = must work
• devas esti preta = must be ready

You cannot normally leave out esti here.

Because Esperanto often uses the present tense for a current obligation, just like English must.

Ĉiu el ni devas esti preta por la kunveno means that right now, it is our obligation to be ready for the meeting.

The duty exists in the present, even if the meeting is later.

Compare:

• Mi devas foriri nun. = I must leave now.
• Ni devas esti pretaj morgaŭ. = We must be ready tomorrow.

So devas does not mean the action is happening now; it means the obligation exists now.

Por means for in the sense of for the purpose of / in preparation for / intended for.

So preta por la kunveno means ready for the meeting.

This is a very common use:

• Mi preparas min por la ekzameno. = I am preparing for the exam.
• Li taýgas por la laboro. = He is suitable for the job.
• Ŝi estas preta por vojaĝi. = She is ready to travel.

Here, por la kunveno tells us what the readiness is for.

La is the definite article, meaning the.

In Esperanto, la kunveno means the meeting, usually a specific meeting already known from context.

For example, speakers may already know which meeting they are talking about:

• the meeting scheduled for today
• the regular weekly meeting
• a meeting previously mentioned

If the sentence said por kunveno without la, it would sound more like for a meeting in a less specific sense.

Yes, Esperanto word order is fairly flexible, because the grammar endings make the relationships clear.

The normal, neutral order is:

• Ĉiu el ni devas esti preta por la kunveno.

But you could move parts around for emphasis, for example:

• Por la kunveno, ĉiu el ni devas esti preta.
• Ĉiu el ni por la kunveno devas esti preta.

Still, the original version is the most natural and straightforward in most situations.

Yes. That is also correct, but it is slightly different in emphasis.

• Ĉiu el ni devas esti preta por la kunveno.
  Focus: each individual among us

• Ni ĉiuj devas esti pretaj por la kunveno.
  Focus: all of us as a group

In practice, both can often translate to the same English sentence, but Esperanto lets you choose the nuance you want.

Notice the agreement changes in the second version:

• ni ĉiuj = plural
• pretaj = plural

Kunveno comes from the idea of coming together.

It is built from:

• kun- = together
• root related to veni = to come
• -o = noun ending

So kunveno means meeting, gathering, assembly.

This is a useful example of how Esperanto builds words from parts. Once you get used to that, vocabulary becomes much easier to understand and remember.

A rough guide is:

• Ĉiu ≈ CHEE-oo
• el ≈ el
• ni ≈ nee
• devas ≈ DEH-vas
• esti ≈ ES-tee
• preta ≈ PREH-ta
• por ≈ por
• la ≈ la
• kunveno ≈ koon-VEH-no

A few helpful points:

• ĉ sounds like ch in church
• stress in Esperanto is almost always on the second-to-last syllable
• so you get:
  • ĈI-u
  • DE-vas
  • ES-ti
  • PRE-ta
  • kun-VE-no

That regular stress pattern is one of the nice things about Esperanto pronunciation.