La infano montras sian orelon al la kuracisto, kaj poste malfermas la buŝon.

Sian is the reflexive possessive adjective. It means his/her/its/their own, and it refers back to the subject of the same clause.

Here, the subject is la infano, so:

• La infano montras sian orelon = the child shows their own ear

If you used a non-reflexive possessive such as lian or ŝian, it would mean someone else’s ear, not the subject’s own ear.

So sian is used because the ear belongs to the child who is doing the action.

The -n ending marks the direct object in Esperanto.

In this sentence:

• montras sian orelon → the ear is what is being shown
• malfermas la buŝon → the mouth is what is being opened

So both orelo and buŝo become accusative:

• orelo → orelon
• buŝo → buŝon

This is one of the most important patterns in Esperanto: the thing directly affected by the verb usually gets -n.

Because la kuracisto is not the direct object. It is the person receiving the action of showing, so Esperanto uses al for that.

The pattern is:

• montri ion al iu = to show something to someone

So:

• sian orelon = the thing shown
• al la kuracisto = the person it is shown to

That is why kuracisto does not take the direct-object -n here.

Because Esperanto often uses la with body parts when the owner is obvious from the context.

Here, after La infano ... poste malfermas la buŝon, it is naturally understood that the child opens their own mouth.

So la buŝon is perfectly normal.

If you said sian buŝon, that would also be correct, but it would be more explicit:

• malfermas la buŝon = opens the mouth, with ownership understood
• malfermas sian buŝon = opens their own mouth, stated more directly

Both are possible, but la buŝon sounds very natural here.

Because the subject stays the same.

The sentence has two coordinated actions:

• La infano montras sian orelon al la kuracisto
• kaj poste malfermas la buŝon

In the second part, Esperanto can leave out the subject because it is still clearly la infano.

English does this too:

• The child shows his ear to the doctor and then opens his mouth.

We do not need to repeat the child a second time.

Poste means afterwards, later, or then.

In this sentence, it shows the sequence of actions:

1. the child shows the ear to the doctor
2. afterward, the child opens the mouth

So kaj poste means something like and then.

It helps make the order of events clear.

Yes. Malfermas comes from:

• fermi = to close
• malfermi = to open

The prefix mal- often gives the opposite meaning of a word.

So:

• fermita = closed
• malfermita = open

This is a very common Esperanto word-building pattern, and learning it will help you recognize many pairs of opposites.

Yes. Esperanto word order is fairly flexible because grammar is shown by endings and prepositions.

These are all natural:

• La infano montras sian orelon al la kuracisto.
• La infano montras al la kuracisto sian orelon.

Both mean the same thing.

Because:

• sian orelon has -n, so it is clearly the direct object
• al la kuracisto is clearly marked by al

So the order can change for style or emphasis without changing the basic meaning.