En la apoteko ŝi aĉetas medikamenton kaj poste metas la kvitancon en sian sakon.

En means in or inside, so en la apoteko means the action happens in the pharmacy.

Al means to or toward. If she were going there, you might say something like ŝi iras al la apoteko = she goes to the pharmacy.

So here, because she is already there buying something, en is the right choice.

The -n ending marks the direct object in Esperanto.

In ŝi aĉetas medikamenton, the medicine is the thing she buys, so it gets -n.

• ŝi = subject
• aĉetas = buys
• medikamenton = direct object

This is one of the most important uses of -n in Esperanto.

For the same reason: it is the direct object of metas.

In ŝi metas la kvitancon..., the receipt is the thing she puts somewhere, so kvitancon takes -n.

So both nouns below are direct objects in their own clauses:

• aĉetas medikamenton
• metas la kvitancon

This is a very common learner question.

Sia is the reflexive possessive. It refers back to the subject of the clause.

Here the subject is ŝi, so:

• ŝi ... metas la kvitancon en sian sakon = she puts the receipt into her own bag

If you said ŝian sakon, it would normally mean another woman's bag, not the subject's own bag.

So:

• sia = the subject's own
• ŝia = hers, but not reflexive; usually someone else's if the subject is already ŝi

Because with some prepositions, Esperanto uses -n to show movement toward a place.

So:

• en sia sako = in her bag
• en sian sakon = into her bag

In this sentence, the receipt is being moved into the bag, so sakon gets -n.

That means the sentence contains two different -n uses at once:

• kvitancon = direct object
• sakon = direction/motion into

Because possessive words like mia, via, lia, ŝia, sia, nia, ilia normally take the place of la.

So you say:

• la sako = the bag
• sia sako = her own bag

Not usually la sia sako.

As for la kvitancon, it refers to a specific receipt, probably the one from the purchase. And la apoteko refers to a specific pharmacy in the situation being described.

All Esperanto verbs in the present tense end in -as.

So:

• aĉeti = to buy

• aĉetas = buys / is buying

• meti = to put
• metas = puts / is putting

In a sentence like this, English might translate the present tense in different ways depending on context, but Esperanto simply uses -as for present time.

Poste means afterward, then, or later.

It shows the order of actions:

1. she buys the medicine
2. poste she puts the receipt into her bag

So it helps connect the two actions in time.

Not completely. Esperanto word order is fairly flexible because endings show the grammatical roles.

The sentence as written is very natural:

En la apoteko ŝi aĉetas medikamenton kaj poste metas la kvitancon en sian sakon.

But other orders are possible, such as putting ŝi first:

Ŝi aĉetas medikamenton en la apoteko kaj poste metas la kvitancon en sian sakon.

The original version sounds smooth and natural because it starts with the setting: In the pharmacy...

A few useful points:

• ŝ sounds like English sh
  • ŝi sounds like shee
• ĉ sounds like English ch
  • aĉetas begins with ah-cheh-
• c in Esperanto sounds like ts
  • so kvitanco ends with a tso sound
• Stress is usually on the second-to-last syllable

Examples:

• apoteko → a-po-TE-ko
• aĉetas → a-ĈE-tas
• kvitancon → kwi-TAN-tson
• sakon → SA-kon

That regular pronunciation is one of the nice things about Esperanto.