Dum la ŝtormo ni vidis fulmon, kaj post ĝi ni aŭdis laŭtan tondron.

Dum means during or while, depending on context.

In this sentence, Dum la ŝtormo means during the storm.

A useful thing to know:

• dum + noun phrase = during
  • dum la ŝtormo = during the storm
• dum + clause = while
  • dum ni atendis = while we were waiting

So here it introduces a time setting for the rest of the sentence.

La is the definite article, meaning the.

So:

• ŝtormo = a storm / storm
• la ŝtormo = the storm

In this sentence, the speaker is talking about a specific storm, not storms in general, so la is natural.

The -n ending marks the direct object in Esperanto.

Here:

• ni vidis fulmon = we saw lightning
• ni aŭdis laŭtan tondron = we heard loud thunder

The things being seen and heard are the direct objects, so they take -n.

Compare:

• fulmo = lightning

• fulmon = lightning, as the direct object

• tondro = thunder
• tondron = thunder, as the direct object

Because adjectives must agree with the nouns they describe.

Since tondron has:

• -o because it is a noun
• -n because it is a direct object

the adjective must match that, so:

• laŭta tondro = loud thunder
• laŭtan tondron = loud thunder, as a direct object

This agreement is very important in Esperanto.

They refer to two different parts of a storm:

• fulmo = lightning
• tondro = thunder

So the sentence says they first saw the lightning and then heard the thunder.

That reflects the normal real-world order too: you usually see lightning before you hear thunder.

Ĝi means it.

Here, post ĝi means after it, and ĝi refers back to fulmon — the lightning flash/event.

So the idea is:

• we saw lightning
• after that, we heard thunder

In natural English, you might often say after it or after that. Esperanto uses ĝi here to point back to the previously mentioned thing.

Because ĝi is the pronoun for a thing, object, animal (when no sex is being emphasized), or an abstract/non-personal referent.

Here the referent is fulmo (lightning), which is not a person, so ĝi is the correct pronoun.

• li = he
• ŝi = she
• ĝi = it

Because Esperanto often leaves out la when something is not being presented as a specifically identified item.

So:

• ni vidis fulmon = we saw lightning / a flash of lightning
• ni aŭdis laŭtan tondron = we heard loud thunder / a loud thunderclap

Using no article here feels natural because the sentence is simply describing what happened during the storm, not identifying a particular previously known lightning flash or thunderclap.

Both are in the past tense.

In Esperanto:

• -as = present
• -is = past
• -os = future

So:

• vidas = see / sees

• vidis = saw

• aŭdas = hear / hears
• aŭdis = heard

That is why the sentence is describing past events.

Yes, Esperanto word order is fairly flexible.

Post ĝi ni aŭdis laŭtan tondron is a natural order because it emphasizes the time relation first: after it.

You could also say:

• Ni aŭdis laŭtan tondron post ĝi.

That would still be understandable. However, the original version flows well because it clearly sets the sequence of events:

1. Dum la ŝtormo ...
2. ni vidis fulmon
3. kaj post ĝi
4. ni aŭdis laŭtan tondron

Yes, often it could.

• post ĝi = after it
• post tio = after that

In many contexts, post tio may sound a little more like referring to the whole event rather than just the noun itself. In this sentence, both would be understandable.

The original post ĝi is perfectly grammatical and directly points back to fulmo.

These are two special Esperanto letters:

• ŝ is pronounced like English sh
  • ŝtormo sounds roughly like SHTOR-mo
• ŭ is a short w-like sound, usually in diphthongs
  • laŭta sounds roughly like LOW-ta

So:

• ŝtormo ≈ shtormo
• laŭtan ≈ low-tan

Also:

• aŭ sounds roughly like the ow in cow
• aŭdi = to hear

That comma separates two main clauses:

• Dum la ŝtormo ni vidis fulmon
• kaj post ĝi ni aŭdis laŭtan tondron

In Esperanto, punctuation before kaj is sometimes used when the sentence joins longer or clearly separate clauses. It helps readability here.

You may also see sentences without such a comma, especially when the structure is short and simple. So this is more a matter of style and clarity than a special grammar rule.