Kiam fulmo ekbrilis super la rivero, la infanoj eksilentis.

Kiam means when. It introduces a time clause:

• Kiam fulmo ekbrilis super la rivero = When lightning flashed over the river

So the whole sentence has two parts:

1. the when-clause
2. the main clause: la infanoj eksilentis

This works very much like English: When X happened, Y happened.

The structure is:

• Kiam = when
• fulmo = lightning / a lightning flash
• ekbrilis = flashed, suddenly shone
• super la rivero = above the river / over the river

So literally it is something like:

When lightning suddenly flashed over the river...

Esperanto often uses a very straightforward word order here: subject + verb + other information

• fulmo = subject
• ekbrilis = verb

ek- usually shows a sudden beginning of an action, or a quick, abrupt action.

So:

• brili = to shine
• ekbrili = to begin to shine suddenly, to flash

And:

• silenti = to be silent, to keep quiet
• eksilenti = to become silent, to fall silent

This prefix is very common in Esperanto and is especially useful for actions that start suddenly.

In this sentence, it gives a dramatic feeling:

• the lightning flashed
• the children fell silent

Because brilis would simply mean shone or was shining, while ekbrilis suggests a sudden flash or sudden start of shining.

Compare:

• La lumo brilis. = The light was shining.
• La lumo ekbrilis. = The light suddenly lit up / flashed.

With fulmo, ekbrilis is especially natural because lightning is sudden.

Because silentis means was silent, while eksilentis means became silent or fell silent.

Compare:

• La infanoj silentis. = The children were silent.
• La infanoj eksilentis. = The children became silent.

So in this sentence, the idea is not that the children were already quiet. It is that the lightning flash caused them to suddenly stop making noise.

In Esperanto, -is marks the past tense.

So:

• ekbrilis = flashed / began to shine
• eksilentis = fell silent / became silent

Both actions are presented as past events.

This is normal even when one action happens just before another. Esperanto often uses the simple past for both, especially in sentences like When X happened, Y happened.

Because fulmo here is being used in a general or indefinite sense: lightning or a flash of lightning.

Compare:

• fulmo = lightning / a lightning flash
• la fulmo = the lightning, a specific lightning flash already known from context

In English, we often say simply lightning flashed, without the, and Esperanto works similarly here.

Because super can describe either:

1. location: above/over something
2. movement toward/onto/over something

Here it describes where the lightning flashed: over the river. That is a location, so Esperanto uses:

• super la rivero

If there were clear movement toward a destination, you might see the accusative:

• super la riveron

But in this sentence, the lightning is not moving to the river as a goal. It is simply flashing above it.

Because -j marks the plural in Esperanto.

• infano = child
• infanoj = children

So:

• la infanoj = the children

This is one of the most basic Esperanto endings:

• noun: -o
• plural: -oj

la means the.

So:

• infanoj = children
• la infanoj = the children

Using la suggests we are talking about a specific group of children, not children in general. In a story, this is very common, because the children are usually known in the situation.

Yes, it is one word, and it is built very regularly:

• ek- = sudden beginning
• silent- = be silent
• -is = past tense

So:

ek + silent + is = eksilentis

This is a good example of how Esperanto builds meaning from parts. Once you know the pieces, long words become much easier to understand.

Yes. Esperanto allows that very naturally.

You could also say:

La infanoj eksilentis, kiam fulmo ekbrilis super la rivero.

That means the same thing: The children fell silent when lightning flashed over the river.

Putting the Kiam clause first often gives a slightly more narrative or dramatic feel, but both are correct.

The comma separates the introductory kiam-clause from the main clause:

• Kiam fulmo ekbrilis super la rivero,
• la infanoj eksilentis.

This is similar to English punctuation:

When lightning flashed over the river, the children fell silent.

It helps the reader see the structure clearly.

Yes. Depending on context, fulmo can be understood as:

• lightning
• a flash of lightning
• sometimes a lightning bolt

In this sentence, the exact English choice depends on style, but the grammar stays the same. The important point for a learner is that fulmo is the thing performing the action ekbrilis.

The sentence sounds vivid and sudden because of the two ek- verbs:

• ekbrilis = lightning suddenly flashed
• eksilentis = the children suddenly fell silent

So grammatically, the sentence is simple, but stylistically it is strong and dramatic. Esperanto often uses ek- to create exactly this kind of quick, sudden scene.