Kiam la kelnerino alportas la fakturon, mia amikino pagas per karto por ĉiuj.

Kiam means when. Here it introduces a time clause:

• Kiam la kelnerino alportas la fakturon = When the waitress brings the bill

So the whole sentence has two parts:

1. the when-clause
2. the main clause: mia amikino pagas per karto por ĉiuj

Esperanto often uses kiam exactly like English when in this kind of sentence.

The ending -in- marks a female person.

• kelnero = waiter / server
• kelnerino = waitress / female server

So la kelnerino specifically means the waitress.

You can also notice:

• amiko = friend
• amikino = female friend

This sentence uses two feminine nouns:

• kelnerino = waitress
• amikino = female friend

The base word is porti = to carry.

The prefix al- often adds the idea of toward or to someone/somewhere.

So:

• porti = to carry
• alporti = to bring, to carry to someone

That is why alportas la fakturon means brings the bill.

It is a very common pattern in Esperanto: a prefix slightly changes the meaning of a basic root.

The -n ending marks the direct object.

Here, the thing being brought is the bill:

• la kelnerino alportas la fakturon
• The waitress brings the bill

So fakturon gets -n because it is the direct object of alportas.

Without -n, the grammar would be wrong in standard Esperanto.

Pagas is present tense, and Esperanto often uses the present tense in a few ways that may feel broader than English:

• for something happening now
• for something habitual
• for vivid narration of a typical situation

So this sentence can mean something like:

• When the waitress brings the bill, my friend pays by card for everyone.

It sounds like a normal recurring or typical event, or a present-tense narration.

If you said pagos, that would clearly mean will pay in the future.

Per means by means of, by using, or via.

So:

• per karto = by card / using a card

This is how Esperanto commonly expresses the means or instrument used to do something.

Other examples:

• skribi per krajono = to write with a pencil
• iri per buso = to go by bus

So pagas per karto literally means pays by means of a card.

Por ĉiuj means for everyone.

It shows who benefits from the action, or on whose behalf the payment is made:

• mia amikino pagas por ĉiuj = my friend pays for everyone

So the meaning is not that she pays everyone, but that she covers the bill for the whole group.

That is why por is important here.

Because ĉiuj comes after the preposition por.

Normally, nouns and pronoun-like words after a preposition do not take -n:

• por ĉiuj = for everyone
• kun amikoj = with friends
• al la urbo = to the city

The accusative -n is mainly for direct objects, and ĉiuj here is not a direct object. It is part of the prepositional phrase por ĉiuj.

In this context, fakturo means the bill or the check at a restaurant.

Even if English invoice and bill can differ by context, Esperanto fakturo can be used for the amount due. In a restaurant sentence like this, the intended meaning is clearly the bill/check.

So learners should understand it here as the thing the waitress brings at the end of the meal.

La is the definite article, meaning the.

• la kelnerino = the waitress
• la fakturon = the bill

It is used because both are specific in the situation:

• the particular waitress serving them
• the particular bill for that table

Esperanto has only one definite article: la. It does not change for gender, number, or case, although the noun may still take -n:

• la fakturo
• la fakturon

Yes. Esperanto word order is fairly flexible because endings show the grammatical roles.

For example, these are possible:

• Kiam la kelnerino alportas la fakturon, mia amikino pagas per karto por ĉiuj.
• Mia amikino pagas per karto por ĉiuj, kiam la kelnerino alportas la fakturon.

Both are understandable.

However, the original order is very natural because it sets the scene first:

• When the waitress brings the bill, ...

That structure is common and smooth in both Esperanto and English.

Yes. Amikino specifically means female friend.

It is built from:

• amik- = friend
• -in- = female
• -o = noun ending

So mia amikino means my female friend.

If the friend were male, you would normally say mia amiko. If gender were not important, many speakers also use amiko in a general sense, depending on style and context.