Ni devas agi nun.

• Ni = we. It is the subject.
• devas = must / have to. This is the finite verb, marked with -as for present tense.
• agi = to act / to take action. This is the infinitive, marked with -i.
• nun = now. It is an adverb of time.

So the structure is basically:

subject + finite verb + infinitive + adverb

Because devas is the verb form that agrees with the sentence as the main verb in the present tense.

In Esperanto:

• devi = to have to / to must, the dictionary form
• devas = have to / must, present tense

So:

• Ni devas = we must
• not Ni devi, because devi is not a conjugated form

Esperanto verbs always use very regular endings:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = imperative / jussive
• -i = infinitive

Because after devas, Esperanto normally uses the infinitive.

So:

• devas agi = must act
• literally, must to act, though Esperanto does not use a separate word for to here

If you said Ni devas agas, that would be wrong, because you would be using two finite verbs together.

The pattern is:

• Mi volas iri = I want to go
• Ŝi povas veni = She can come
• Ni devas agi = We must act

Not in this sentence.

English says must act, not must to act, and Esperanto works similarly here. The infinitive ending -i already shows that agi means to act.

So agi by itself already carries the infinitive idea.

That is why:

• agi = to act
• devas agi = must act

devas usually expresses fairly strong necessity: must or have to.

So Ni devas agi nun means something like:

• We must act now
• We have to act now

It is stronger than should.

If you want a softer idea like should, Esperanto often uses:

• Ni devus agi nun = We should act now

So the difference is:

• devas = must / have to
• devus = should / ought to

Usually in a sentence like this, agi means to act, to take action, or to do something, not to perform as an actor.

So Ni devas agi nun most naturally means:

• We must take action now

If you specifically wanted to act in a play, the context would normally make that clear. But without special context, learners should understand agi here as take action.

Yes. Esperanto word order is fairly flexible, especially with adverbs like nun.

These are all possible:

• Ni devas agi nun
• Nun ni devas agi
• Ni nun devas agi

They all mean roughly the same thing, but the emphasis can shift a little.

• Ni devas agi nun = neutral, very common
• Nun ni devas agi = emphasizes now
• Ni nun devas agi = also emphasizes the timing somewhat

This order is natural and very common, and it happens to match English closely:

We + must + act + now

Esperanto often uses a straightforward subject–verb–other elements order, but it is more flexible than English because grammar is shown clearly by endings.

So this sentence could be rearranged for emphasis, but the standard order is easy and natural:

• Ni devas agi nun

For a beginner, this is the safest default pattern.

A simple pronunciation guide is:

nee DEH-vahs AH-ghee noon

A few details:

• Ni sounds like nee
• devas has stress on DE
• agi has stress on A
• nun rhymes roughly with noon

In Esperanto, stress is almost always on the second-to-last syllable:

• DE-vas
• A-gi

That regular stress rule is very helpful.

Because agi does not need a direct object here.

In this sentence, agi simply means to act or to take action, and that idea is complete by itself.

English works the same way:

• We must act now

You do not have to say what action is being taken unless you want to add more detail.

If you wanted, you could expand it:

• Ni devas agi nun por helpi ilin. = We must act now to help them.

But the short sentence is already complete.

Because ni is the subject form.

In Esperanto, -n is often added for the direct object:

• ni = we
• nin = us

Since we are doing the action, the sentence needs the subject form:

• Ni devas agi nun

If the pronoun were an object, then nin could appear in a different sentence, for example:

• Ili vidas nin. = They see us.

Yes. That is one of the nice regular features of Esperanto.

You can change the tense of the main verb devi very easily:

• Ni devis agi nun. = We had to act now
• Ni devos agi nun. = We will have to act now
• Ni devus agi nun. = We should act now

Notice that agi stays in the infinitive. The tense or mood is shown on devi, not on agi.