Kiam la leciono estos finita, la tabulo estos pura, kaj la klaso denove fariĝos silenta.

Because fini is normally a transitive verb in Esperanto: it means to finish something.

So:

• Mi finas la laboron = I finish the work.
• La leciono finos would literally suggest the lesson will finish something, which is not what is meant here.

In la leciono estos finita, the lesson is the thing that has been finished.
finita is a participle meaning finished, so estos finita means will be finished.

Yes. That would also be natural.

The difference is mainly one of focus:

• la leciono estos finita = the lesson will be in a finished state
• la leciono finiĝos = the lesson will come to an end / will finish

In many situations, both work well. The version with estos finita sounds a bit more like emphasizing the completed result.

Esperanto does not follow English here.

In Esperanto, tense usually reflects the actual time of the action more directly. Since the lesson will be finished in the future, estos finita is perfectly normal after kiam.

So:

• Kiam la leciono estos finita... = When the lesson will be finished / When the lesson is finished

Even if English prefers present tense after when, Esperanto can use the future normally.

finita is the past passive participle of fini.

A quick breakdown:

• fin- = finish
• -it- = past passive participle
• -a = adjective ending

So finita means finished or having been finished.

Because participles can act like adjectives in Esperanto, finita agrees with leciono and describes its state.

Because they are all adjectives.

In this sentence:

• finita describes la leciono
• pura describes la tabulo
• silenta describes la klaso

Esperanto adjectives always end in -a.

Also, these are predicate adjectives, used after verbs like esti and fariĝi, so they do not take -n here.

fariĝos comes from fariĝi, which means to become.

So:

• estos silenta = will be silent
• fariĝos silenta = will become silent

This is an important difference.
fariĝos shows a change of state, not just a state itself.

Here, the class is not simply described as silent; it is said to become silent again.

Both are possible in Esperanto.

• fariĝos silenta = will become silent
• silentiĝos = will fall silent / will become silent

The version in the sentence is very clear and easy to understand, especially for learners.
It uses the common pattern fariĝi + adjective.

silentiĝos is a bit more compact, but the meaning is very similar.

denove means again.

In this sentence, it means that the class will become silent once more.

Its position before fariĝos is natural:

• la klaso denove fariĝos silenta

But Esperanto word order is somewhat flexible, so you may also see:

• la klaso fariĝos denove silenta

Both are understandable. The original version sounds very natural.

Here it most likely means the class as a group of students.

That is the most natural reading in this context: after the lesson is finished, the board is clean, and the class becomes silent again.

If you specifically mean the physical classroom, Esperanto often uses klasĉambro for classroom.

So:

• klaso = class, group of students
• klasĉambro = classroom

Because Esperanto uses la as its definite article, like English the.

Here the speaker is referring to specific things:

• la leciono = the lesson
• la tabulo = the board
• la klaso = the class

Esperanto has only one definite article, la, and it does not change for gender, number, or case. Repeating it before each noun is completely normal.

Not necessarily.

tabulo basically means board. In a school context, it can refer to:

• a blackboard
• a chalkboard
• a whiteboard

The exact type usually depends on context. So la tabulo estos pura simply means the classroom board will be clean.