Kiam la instruistino demandis pri la eraro, la klaso eksilentis.

Kiam means when. Here it introduces a time clause:

• Kiam la instruistino demandis pri la eraro = When the teacher asked about the mistake

Esperanto often puts this kind of subordinate clause first, just like English can:

• When the teacher asked about the mistake, the class fell silent.

You could also put the main clause first, but this order is very natural.

The comma separates the subordinate clause from the main clause.

• Kiam la instruistino demandis pri la eraro = subordinate time clause
• la klaso eksilentis = main clause

Esperanto commonly uses a comma before or after subordinate clauses, especially when the clause comes first.

Instruistino means female teacher.

It is built from:

• instrui = to teach
• instruisto = teacher
• instruistino = female teacher

The suffix -in- marks female sex. So:

• patro = father
• patrino = mother
• instruisto = teacher
• instruistino = female teacher

A learner may notice that English teacher does not show gender, but Esperanto here does.

Yes. Instruisto means teacher and can be used when the teacher’s sex is not being emphasized, or in some contexts as a general form.

But instruistino specifically tells you the teacher is female. The sentence chooses to include that detail.

Because -is is the Esperanto past-tense ending.

• demandi = to ask
• demandas = asks / is asking
• demandis = asked
• demandos = will ask

So la instruistino demandis means the teacher asked.

Pri usually means about, concerning, or regarding.

So:

• demandi pri la eraro = to ask about the mistake

This is a common pattern in Esperanto. The topic being asked about is introduced with pri.

Because it is not a direct object here. It is the object of the preposition pri.

In Esperanto, nouns after prepositions normally do not take -n just because they follow a preposition:

• pri la eraro = about the mistake

The accusative -n is mainly used for direct objects and certain motion expressions, not after ordinary prepositions like pri.

In Esperanto, klaso can mean class in the sense of the group of students. That is exactly how it is used here.

So:

• la klaso eksilentis = the class fell silent

It does not mean the physical room became silent by itself; it refers to the group.

Eksilentis means something like fell silent, went silent, or became suddenly silent.

It is made from:

• silenti = to be silent / to keep silent
• ek- = marks the beginning of an action, often suddenly
• eksilentis = became silent, fell silent

So the idea is not just that the class was silent, but that they suddenly became silent.

Ek- often shows the beginning of an action, sometimes with a sudden feeling.

Examples:

• kanti = to sing → ekkanti = to start singing
• ridi = to laugh → ekridi = to burst out laughing / start laughing
• silenti = to be silent → eksilenti = to fall silent

So eksilentis suggests a change: the class was not silent before, and then suddenly became silent.

Because here Esperanto uses a verb.

• silenta = silent, as an adjective
• silenti = to be silent / remain silent

Then with ek-:

• eksilenti = to become silent / fall silent

This is a very natural Esperanto way to express the idea. English often uses fall silent, while Esperanto can build it neatly with a verb.

Because la klaso is the subject of the main clause.

• la klaso eksilentis = the class fell silent

The class is the thing doing the action, so it stays in the basic form without -n.

Yes, Esperanto word order is fairly flexible, but this version is very natural.

The sentence is organized as:

1. time clause first: Kiam la instruistino demandis pri la eraro
2. main clause second: la klaso eksilentis

This order helps the sentence flow clearly: first the trigger, then the reaction.

Because both nouns are definite:

• la instruistino = the teacher
• la eraro = the mistake
• la klaso = the class

Esperanto uses la for the. It does not change for gender, number, or case.

Yes. Eraro can mean error or mistake, depending on context.

So pri la eraro could be understood as:

• about the mistake
• about the error

In many learning contexts, mistake sounds more natural in English, but both are possible.

No. Like English when, kiam can be used in two ways:

• as a question word: Kiam vi venos? = When will you come?
• as a conjunction introducing a clause: Kiam li alvenis, ni komencis. = When he arrived, we began.

In your sentence, it is the second use: it introduces a time clause, not a direct question.