La infano ekkriis, kiam ŝi aŭdis la unuan tondron de la vespero.

Yes. Infano is normally gender-neutral: it just means child. The pronoun ŝi tells you that this particular child is female.

So in this sentence:

• la infano = the child
• ŝi = she

Esperanto often leaves gender out of the noun unless it matters, and then a pronoun or context makes it clear.

It comes from:

• krii = to cry out, shout
• ek- = a prefix showing the beginning of an action, or a sudden action
• -is = past tense

So ekkriis means something like cried out, let out a cry, or suddenly shouted.

It is not just was shouting. It focuses on the sudden start of the cry.

Because Esperanto keeps both parts unchanged:

• ek-
  • krii = ekkrii
• past tense: ekkriis

The k at the end of the prefix and the k at the beginning of the root both stay. So the double k is normal and expected.

Because aŭdi means to hear, while aŭskulti means to listen.

That is the same distinction as in English:

• hear = perceive a sound
• listen = actively pay attention to a sound

Here the child did not choose to listen to the thunder; she simply heard it. So aŭdis is the right verb.

Because both actions happened in the past.

• ekkriis = cried out
• aŭdis = heard

The word kiam shows the time relationship: the crying out happened when she heard the thunder. Esperanto simply marks each verb for the time it happened; here both are past events in a narrative.

Here kiam introduces a subordinate time clause, meaning when.

So:

• kiam ŝi aŭdis... = when she heard...

Kiam can also be used in a question, as in Kiam vi venos? = When will you come? But in this sentence it is not a question word; it is a conjunction introducing a clause.

Because kiam ŝi aŭdis la unuan tondron de la vespero is a subordinate clause, and Esperanto normally separates such clauses with a comma.

So the structure is:

• main clause: La infano ekkriis
• subordinate clause: kiam ŝi aŭdis la unuan tondron de la vespero

This kind of comma is very common in Esperanto.

Because unu means one, but unua means first.

Here the sentence is talking about the first thunder of the evening, not just one thunder. So it needs the ordinal form:

• unu = one
• unua = first

Then it becomes unuan because of the accusative ending -n.

Because la unuan tondron is the direct object of aŭdis.

In Esperanto, the direct object takes -n:

• ŝi aŭdis tondron = she heard thunder

And adjectives agree with the noun they describe, so unua must match tondro:

• singular: yes
• accusative: yes

That gives:

• unuan tondron

Tondro means thunder. In context, it can also feel like a thunderclap or a rumble of thunder.

Because the sentence says la unuan tondron, English often translates it more naturally as the first thunderclap or the first peal of thunder, since it is treating thunder as a countable event in that moment.

De often means of. So la unua tondro de la vespero means the first thunder of the evening.

This phrase identifies which first thunder we mean: the first one that happened during that evening.

A phrase like vespera tondro would mean evening thunder, but de la vespero more clearly expresses the first thunder belonging to that evening's sequence.

Because la is Esperanto’s definite article, meaning the.

• la infano = the child
• la vespero = the evening

Esperanto has no separate word for a/an, so:

• infano can mean a child or just child, depending on context
• la infano specifically means the child

In this sentence, both the child and the evening are treated as specific ones in the story or situation.

Yes. Esperanto word order is fairly flexible, so you could also say:

Kiam ŝi aŭdis la unuan tondron de la vespero, la infano ekkriis.

That means the same thing. The version with the main clause first is just one natural way to present it.