La terapiisto fleksas la kubuton antaŭ ol masaĝi ĝin.

Because la kubuton is the direct object of fleksas.

• fleksi ion = to bend/flex something
• so la terapiisto fleksas la kubuton = the therapist bends the elbow

In Esperanto, direct objects usually take -n:

• kubuto = elbow
• kubuton = elbow as the object of the verb

The same idea appears in ĝin later in the sentence: it also has the accusative -n because it is the object of masaĝi.

Ĝin means it, and it refers back to la kubuton.

So:

• masaĝi ĝin = to massage it
• it = the elbow

Esperanto often uses a pronoun like this instead of repeating the noun:

• La terapiisto fleksas la kubuton antaŭ ol masaĝi la kubuton.
  This is grammatical, but repetitive.
• La terapiisto fleksas la kubuton antaŭ ol masaĝi ĝin.
  This sounds more natural.

Here masaĝi is an infinitive, meaning to massage.

After antaŭ ol, Esperanto often uses an infinitive when the subject is understood and does not need to be repeated:

• antaŭ ol masaĝi ĝin = before massaging it / before to massage it

In natural English, we usually say before massaging it or before he/she massages it.

A full clause is also possible when you want to state the subject clearly:

• ... antaŭ ol ŝi masaĝas ĝin = before she massages it

So the infinitive is a compact way to say the action without repeating the subject.

Antaŭ ol means before when it introduces an action.

Compare:

• antaŭ la tablo = in front of / before the table
  (antaŭ
  • noun phrase)
• antaŭ ol masaĝi ĝin = before massaging it
  (antaŭ ol
  • verb/action)

So in this sentence, antaŭ ol links two actions:

1. the therapist bends the elbow
2. then the therapist massages it

It shows that the first action happens earlier.

Because antaŭ by itself is normally a preposition, and prepositions usually go before a noun phrase, not directly before a bare verb.

So Esperanto normally uses:

• antaŭ la masaĝo = before the massage
• antaŭ ol masaĝi ĝin = before massaging it

The ol is what allows antaŭ to connect to an action or clause.

Because both are being treated as specific:

• la terapiisto = the therapist
• la kubuton = the elbow

Esperanto uses la much like English the.

Also, with body parts, Esperanto often uses la when the owner is obvious from context, instead of a possessive word like his/her.

For example:

• Li levas la manon. = He raises his hand.
• Ŝi fermas la okulojn. = She closes her eyes.

So la kubuton can naturally mean the elbow in the sense of the relevant elbow, without explicitly saying whose elbow it is.

No, not explicitly.

La kubuton just means the elbow. The sentence itself does not grammatically mark whether it is:

• the therapist’s elbow
• the patient’s elbow
• someone else’s elbow

Usually context tells you. In a medical or therapy context, readers would probably assume it is the patient’s elbow.

If you wanted to be explicit, you could say something like:

• la kubuton de la paciento = the patient’s elbow
• ŝian kubuton = her elbow
• lian kubuton = his elbow

The suffix -ist- usually means a person associated with a profession, activity, or ideology.

So:

• terapio = therapy
• terapiisto = therapist / therapy practitioner

Other common examples:

• piano → pianisto = pianist
• arto → artisto = artist
• scienco → sciencisto = scientist

So a learner can often recognize -isto as a person who does or works with something.

Yes. Fleksi means to flex, bend.

In a medical or anatomical context, fleksi la kubuton is a very natural way to say bend the elbow or flex the elbow.

So:

• fleksas = bends / flexes
• la terapiisto fleksas la kubuton = the therapist bends/flexes the elbow

The exact English wording may vary, but the Esperanto is straightforward.

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

For example, you could also say:

• Antaŭ ol masaĝi ĝin, la terapiisto fleksas la kubuton.

This means the same thing: Before massaging it, the therapist bends the elbow.

The -n on kubuton and ĝin helps show what the objects are, so the sentence can be rearranged more easily than in English.

The letter ĝ is pronounced like the s in measure or the g in genre.

So roughly:

• masaĝi ≈ ma-SA-zhi
• ĝin ≈ zheen

A few other pronunciation points:

• Stress in Esperanto usually falls on the second-to-last syllable
• so te-ra-pi-IS-to
• KU-bu-ton
• ma-SA-ĝi

That regular stress pattern makes Esperanto pronunciation very predictable.