Ŝi fleksas la genuon sed ne la maleolon.

Ŝi means she.

Si is a special reflexive pronoun meaning himself / herself / itself / themselves, and it is only used when the object refers back to the subject of the same clause.

So here:

• Ŝi fleksas la genuon = She bends the knee
• Ŝi fleksas sin would mean She bends herself

Since the subject is simply she, Esperanto uses ŝi.

Fleksas comes from the verb fleksi, meaning to bend or to flex.

The ending -as shows the present tense:

• fleksi = to bend
• fleksas = bends / is bending / does bend
• fleksis = bent
• fleksos = will bend

Esperanto verbs do not change according to the subject, so:

• mi fleksas = I bend
• ŝi fleksas = she bends
• ili fleksas = they bend

The verb form stays the same.

The -n ending marks the direct object in Esperanto.

In this sentence, the things being bent are:

• la genuon = the knee
• la maleolon = the ankle

They receive the action of fleksas, so they take -n.

Compare:

• La genuo doloras. = The knee hurts.
  (Here genuo is the subject, so no -n.)
• Ŝi fleksas la genuon. = She bends the knee.
  (Here genuon is the object, so it gets -n.)

La means the.

Esperanto uses la when the noun is definite. In many body-part sentences, Esperanto often uses la where English might use her or just the, depending on style.

So la genuon is natural Esperanto for the knee.

With body parts, Esperanto often does not need a possessive like ŝian if ownership is obvious from the context:

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

You could say ŝian genuon if you really wanted to emphasize that it is her knee, but usually la genuon is enough.

It is repeated because la maleolon is understood as a second object parallel to la genuon.

The sentence is basically:

• Ŝi fleksas la genuon sed ne fleksas la maleolon.

The second fleksas is omitted because it is understood, but the object still keeps its own article:

• la genuon
• la maleolon

Repeating la makes the structure clear and natural.

Sed means but, and ne means not.

Together, sed ne means something like:

• but not
• but does not

Here it contrasts two things:

• she bends the knee
• not the ankle

So the full sense is:

• She bends the knee, but not the ankle
• or more fully: She bends the knee but does not bend the ankle

Yes. Esperanto often omits repeated words when they are obvious.

So:

• Ŝi fleksas la genuon sed ne la maleolon.

is a shortened form of:

• Ŝi fleksas la genuon sed ne fleksas la maleolon.

This kind of omission is common and natural. English does something similar:

• She bends the knee but not the ankle.

They are two different body parts:

• genuo = knee
• maleolo = ankle

Both are nouns, and both take -n here because they are direct objects:

• genuon
• maleolon

If you want the basic forms, remove the accusative ending:

• genuon → genuo
• maleolon → maleolo

Even though it begins with mal-, maleolo is generally learned as a whole word meaning ankle.

A learner might guess it is:

• mal-
  • something

but in practice, maleolo is just the standard vocabulary item for ankle. It is not usually analyzed in the same productive way as clear examples like:

• bona = good
• malbona = bad

So it is best to memorize maleolo as its own word.

Grammatically, the singular forms genuon and maleolon mean one knee and one ankle.

However, in normal usage, the sentence may simply be talking about the body part in a general or situational way, without stressing which specific one.

If the speaker wanted to be more explicit, they could say:

• la dekstran genuon = the right knee
• la maldekstran maleolon = the left ankle

But as written, it is singular and refers to one knee and one ankle.

A rough pronunciation guide:

• Ŝi ≈ shee
• fleksas ≈ FLEK-sahs
• la ≈ lah
• genuon ≈ geh-NOO-on
• sed ≈ sed
• ne ≈ neh
• la ≈ lah
• maleolon ≈ mah-leh-O-lon

A few helpful points:

• ŝ sounds like English sh
• g is always a hard g
• each vowel is pronounced clearly
• stress in Esperanto normally falls on the second-to-last syllable

So:

• ge-NU-o → stress on NU
• ma-le-O-lo → stress on O

With the accusative -n, you still keep the same stress pattern:

• ge-NU-on
• ma-le-O-lon