Kiam la glaso estas malplena, la infano petas min plenigi ĝin per akvo.

Kiam means when. It introduces a time clause: Kiam la glaso estas malplena = When the glass is empty.

In this sentence, kiam means whenever / when in a general sense, not necessarily one single specific moment.

Esperanto usually separates a subordinate clause from the main clause with a comma.

So here:

• Kiam la glaso estas malplena = subordinate clause
• la infano petas min plenigi ĝin per akvo = main clause

The comma helps show that the first part sets the situation for the second part.

La means the. Esperanto has only one article: la. It does not have a separate word for a/an.

So:

• la glaso = the glass
• la infano = the child

The sentence uses la because it is talking about a specific glass and a specific child, or at least ones understood from the context.

Malplena means empty.

It is built from:

• plena = full
• mal- = a prefix meaning the opposite

So:

• plena = full
• malplena = empty

This is very common in Esperanto. The prefix mal- often creates the opposite meaning:

• bona = good
• malbona = bad

In Esperanto, adjectives are commonly used with esti (to be), just like in English.

So:

• estas = is
• malplena = empty

Together:

• la glaso estas malplena = the glass is empty

This works just like English is empty.

The ending -a marks an adjective in Esperanto.

Since malplena describes glaso, it must be an adjective:

• glaso = glass
• malplena = empty

Esperanto endings are very regular:

• -o = noun
• -a = adjective
• -e = adverb
• -i = infinitive verb

Because peti is a transitive verb here, and min is its direct object.

The pattern is:

• peti iun fari ion = to ask someone to do something

So:

• petas min = asks me
• plenigi ĝin = to fill it

Altogether:

• la infano petas min plenigi ĝin = the child asks me to fill it

The -n on min shows that I am the object of petas.

In Esperanto, the infinitive already ends in -i, so you usually do not need a separate word like English to.

So:

• plenigi = to fill

After verbs like peti, Esperanto often uses the infinitive directly:

• petas min plenigi ĝin = asks me to fill it

There is no need for an extra word corresponding to English to.

Plena is an adjective meaning full.

Plenigi is a verb meaning to make full, so in natural English, to fill.

It is formed with the suffix -ig-, which means to cause to become or to make.

So:

• plena = full
• plenigi = to make full / to fill

This is a very useful pattern in Esperanto.

Because ĝin is the direct object of plenigi.

• ĝi = it
• ĝin = it (accusative)

Here, the thing being filled is the glass, so Esperanto uses the accusative form:

• plenigi ĝin = to fill it

The pronoun ĝin refers back to la glaso.

Per usually means by means of, using, or with in the sense of the material or instrument used.

So:

• per akvo = with water / using water

This is the natural choice because the water is what is used to fill the glass.

By contrast, kun usually means with in the sense of together with or accompanied by.

So in this sentence:

• plenigi ĝin per akvo = fill it with water

Because akvo is not the direct object here.

The direct object of plenigi is ĝin:

• plenigi ĝin = fill it

Per akvo is a prepositional phrase, and nouns after prepositions normally do not take -n just because they come after a preposition.

So:

• ĝin gets -n
• akvo does not

The sentence has this structure:

• Kiam la glaso estas malplena = time clause
• la infano petas min = main clause
• plenigi ĝin per akvo = infinitive phrase explaining what the child asks me to do

So the full structure is:

When the glass is empty, the child asks me to fill it with water.

This is a very normal and natural Esperanto word order.