Kiam la kelnerino venos, mi mendos supon kaj akvon.

Because -os is the Esperanto ending for the future tense.

• venos = will come
• mendos = will order

So the sentence talks about two future actions:

1. the waitress will come
2. then I will order soup and water

Unlike English, Esperanto uses the normal future ending in both parts.

Kiam means when.

In this sentence, it introduces a time clause:

• Kiam la kelnerino venos = When the waitress comes / will come

So kiam connects the two actions by time: first the waitress arrives, then the speaker orders.

The comma separates the introductory time clause from the main clause.

• Kiam la kelnerino venos, = subordinate clause
• mi mendos supon kaj akvon. = main clause

This is very common in Esperanto, especially when the subordinate clause comes first.

The suffix -ino marks a female person.

• kelnero = waiter
• kelnerino = waitress

So kelner- is the root related to serving in a restaurant, and -ino tells you the person is female.

La is the definite article, meaning the.

So:

• la kelnerino = the waitress
• kelnerino = a waitress / waitress depending on context

Esperanto has la for the, but it does not have a special word for a/an. If you leave out la, the meaning is usually more indefinite.

Here, la kelnerino suggests a specific waitress, probably the one serving the table.

The -n ending marks the direct object in Esperanto.

The direct object is the thing receiving the action of the verb. Here, the action is mendos (will order), and the things being ordered are:

• supon = soup
• akvon = water

So the speaker is ordering those things, which is why they take -n.

Without the -n, the sentence would be grammatically wrong.

Because kaj just means and. It connects two nouns, but each noun still keeps the form it needs.

So:

• supon = direct object
• akvon = direct object

Both are objects of mendos, so both need -n.

You cannot usually put the -n only on the last one and expect it to cover both. Standard Esperanto keeps the marking on each object:

• supon kaj akvon

Formally, yes, the Esperanto verb is in the future: venos = will come.

But in natural English, after when, we usually say:

• When the waitress comes, I will order...

not

• When the waitress will come...

Esperanto does not follow that English pattern here. It simply uses the tense that matches the meaning: the coming is in the future, so venos is used.

That is a very common thing for English speakers to notice.

Yes, Esperanto word order is fairly flexible because endings show the grammar.

For example, these are possible:

• Kiam la kelnerino venos, mi mendos supon kaj akvon.
• Mi mendos supon kaj akvon, kiam la kelnerino venos.

Both mean essentially the same thing.

However, the original version is very natural because it puts the time condition first.

Esperanto does not have an indefinite article like English a/an.

So:

• supon can mean soup or a soup, depending on context
• akvon can mean water or some water

Esperanto only has the definite article la for the. If no la appears, the noun is usually indefinite or general.

Yes. Mendi means to order, including in a restaurant.

So:

• mendi supon = to order soup
• mendi akvon = to order water

This is a very normal use of the verb.

By itself, akvon just means water as the thing being ordered.

Esperanto often leaves details like this to context, just like English often does. If you wanted to be more specific, you could say something like:

• glason da akvo = a glass of water
• botelon da akvo = a bottle of water

But in a simple restaurant sentence, akvon is completely natural.