Kiam la luno estas tre hela, nia avino rigardas ĝin de la balkono.

Kiam means when and introduces a time clause.

So Kiam la luno estas tre hela means when the moon is very bright. That whole part sets the time or situation for the main clause nia avino rigardas ĝin de la balkono.

Esperanto often puts this kind of time clause first, just as English can.

The comma separates the subordinate clause from the main clause.

• Kiam la luno estas tre hela = subordinate time clause
• nia avino rigardas ĝin de la balkono = main clause

This is very normal in written Esperanto, especially when the subordinate clause comes first. It helps readability.

La is the definite article, meaning the.

In this sentence:

• la luno refers to a specific, identifiable moon
• la balkono refers to a specific balcony, presumably the one relevant to the family or situation

Esperanto uses la when the noun is definite, much like English the.

Because hela is an adjective describing la luno.

After estas, Esperanto normally uses an adjective for a quality of the subject:

• la luno estas hela = the moon is bright

If you said hele, that would be an adverb, which describes how something happens, not what something is like. So hele would not fit here.

In Esperanto, possessives like mia, via, nia usually make the noun definite by themselves, so you normally do not add la.

So:

• nia avino = our grandmother

Using la here would usually sound wrong in ordinary Esperanto. This is similar to English, where we say our grandmother, not the our grandmother.

Because ĝin is the accusative form, marked by -n, and it is the direct object of rigardas.

• ĝi = it
• ĝin = it as a direct object

Here, grandmother is looking at the moon, so the moon is the object of the action.

Because rigardi is a transitive verb in Esperanto. It directly takes an object.

So Esperanto says:

• rigardi ion = to look at something

English uses the preposition at, but Esperanto does not need one here. This is a very common thing for English speakers to notice: Esperanto often uses a direct object where English uses a verb plus preposition.

In normal Esperanto, things and animals of unspecified sex are usually referred to with ĝi.

So the moon is treated grammatically as a thing:

• la luno ... ĝin

Esperanto does not assign grammatical gender to nouns the way some European languages do. Even if a language or poetic tradition treats the moon as feminine or masculine, standard Esperanto simply uses ĝi for an object like this.

Here de la balkono means something like from the balcony or from the vantage point of the balcony.

It emphasizes the place from which the grandmother is looking. Depending on context, Esperanto could also use other prepositions:

• sur la balkono = on the balcony
• el la balkono = out of/from inside the balcony area in some contexts

So de la balkono focuses more on the point of origin of the looking, while sur la balkono would focus more directly on physical location.

The present tense here expresses a general or habitual idea:

• Whenever the moon is very bright, our grandmother looks at it from the balcony.

So this is not necessarily one event happening right now. It can describe something she regularly does under those circumstances. Esperanto present tense often works this way, just like English present tense in general statements.

Yes. Esperanto word order is fairly flexible, as long as the grammar stays clear.

For example, you could also say:

• Nia avino rigardas ĝin de la balkono, kiam la luno estas tre hela.

That still makes sense. The original version puts the when clause first, which feels natural because it sets the scene before the main action.