Kiam ektondris denove, la hundo kuris sub la tablon, kaj subite ne plu estis elektro en la domo.

Ektondris is made from:

• tondri = to thunder
• ek- = a prefix meaning to begin suddenly or to start doing something
• -is = past tense

So ektondris means started thundering or thundered suddenly.

In this sentence, Kiam ektondris denove is basically When it started thundering again.

In Esperanto, weather verbs often do not need a dummy subject like English it.

English says:

• It thundered
• It rained

But Esperanto can simply say:

• Tondris
• Pluvis

So Kiam ektondris denove is normal Esperanto, even though English would usually need it.

Denove means again.

So:

• ektondris = started thundering
• ektondris denove = started thundering again

It tells you that the thunder happened another time, not just once.

This is a very common Esperanto point.

After a preposition, Esperanto sometimes uses the -n ending to show movement toward a place, not just location.

So:

• sub la tablo = under the table, as a location
• sub la tablon = to a position under the table

In the sentence, the dog ran to under the table, so sub la tablon is correct.

Compare:

• La hundo estas sub la tablo. = The dog is under the table.
• La hundo kuris sub la tablon. = The dog ran under the table.

Because la hundo is the subject of kuris.

In Esperanto, the direct object gets -n, but the subject does not.

Here:

• la hundo = the dog → subject
• kuris = ran

So the dog is the one doing the action.

By contrast, la tablon has -n, but not because it is a direct object of kuri. It has -n because of direction/movement after sub.

Ne plu means no longer.

So:

• plu by itself often means further / more / any more
• ne plu = not anymore / no longer

Therefore:

• ne plu estis elektro = there was no longer electricity

This is a very common Esperanto expression.

Usually, learners should use ne plu.

That is the normal and standard way to say no longer.

So:

• Mi ne plu fumas. = I no longer smoke.
• Ne plu estis elektro. = There was no longer electricity.

Using plu ne would sound unusual and is generally not what learners should aim for here.

Estis elektro is an existence-style construction: it means that electricity was present/available.

So:

• estis elektro en la domo = there was electricity in the house
• ne plu estis elektro en la domo = there was no longer electricity in the house

This is similar to English there was.

If you say la elektro, that sounds more like you are talking about the electricity as a specific known thing. Here the sentence is just saying that electrical power was no longer available.

Yes. In context, elektro can mean electricity or electrical power.

So ne plu estis elektro en la domo means the house lost electrical power.

You may also encounter kurento in similar situations, especially when talking about electric current or power supply in everyday use. But elektro is understandable here.

Subite means suddenly.

Esperanto word order is fairly flexible, so adverbs like subite can often move around depending on emphasis.

Here:

• kaj subite ne plu estis elektro en la domo

means:

• and suddenly there was no longer electricity in the house

Putting subite early in the clause makes the suddenness stand out.

Kiam means when and introduces a time clause.

So:

• Kiam ektondris denove = When it started thundering again

This clause sets the time for what follows:

• the dog ran under the table
• then the electricity was gone

Esperanto often uses the same kind of structure as English here: When X happened, Y happened.

The first comma separates the time clause from the main clause:

• Kiam ektondris denove, la hundo kuris sub la tablon

That is like English:

• When it started thundering again, the dog ran under the table

The second comma before kaj is a stylistic pause between two full clauses:

• la hundo kuris sub la tablon
• kaj subite ne plu estis elektro en la domo

In Esperanto, punctuation can be somewhat flexible, but these commas are completely natural.

Yes, but it would mean something slightly different.

• iris sub la tablon = went under the table
• kuris sub la tablon = ran under the table

Kuris gives a stronger sense of speed or fear, which fits the situation well: the thunder starts again, and the dog quickly runs for cover.

En means in or inside.

So:

• en la domo = in the house

That tells us where the electricity was no longer available.

A learner might compare:

• en la domo = inside the house
• ĉe la domo = at the house / by the house

Here, en la domo is the correct choice because the meaning is about power inside the house.

La is the definite article, meaning the.

In this sentence, it refers to specific things in the situation:

• la hundo = the dog
• la tablon = the table
• la domo = the house

Whether Esperanto uses la can depend on context, just as English does. Here, all three are presented as specific, identifiable things, so la is natural.