Etendu kaj poste fleksu la genuon.

Because -u is the Esperanto verb ending for a command, request, or instruction.

So:

• etendi = to extend
• etendu = extend!
• fleksi = to flex/bend
• fleksu = flex!/bend!

In this sentence, the speaker is giving an instruction, so -u is the natural ending.

Yes. In Esperanto, commands often leave out the subject, just like English often does.

So the sentence is understood as being directed to you:

• implied vi = you

A fuller version would be:

• Vi etendu kaj poste fleksu la genuon.

But in normal instructions, especially exercise or medical directions, leaving out vi is more natural.

The -n marks the direct object.

Here, la genuon is the thing being extended and flexed. That makes it the direct object of the verbs.

• genuo = knee
• genuon = knee as a direct object

Esperanto uses -n very regularly for direct objects, so this is a very common pattern.

Esperanto often uses the article with body parts when it is obvious whose body part is meant.

So la genuon here naturally means the knee in the sense of your knee, because the instruction is being given directly to someone.

Using vian genuon is possible, but it sounds more explicit or emphatic:

• la genuon = the knee / your knee, when obvious from context
• vian genuon = your knee, stated directly

This is similar to how some languages prefer the with body parts instead of always using a possessive.

Because the sentence refers to one knee.

If the instruction were about both knees, you would use the plural:

• la genuojn

So the singular suggests one knee at a time. In exercise or medical instructions, that is very common.

Poste is an adverb meaning afterward, after that, or then.

It shows the order of the actions:

1. etendu
2. poste fleksu

Without poste, the sentence would still connect the two actions, but poste makes the sequence especially clear.

They do slightly different jobs:

• kaj = and
• poste = then / afterward

So kaj poste means and then.

Using both is completely natural because:

• kaj links the two commands
• poste shows that the second one happens after the first

If you removed poste, the sentence would still work, but the step-by-step order would be less explicit.

Because both verbs share the same object.

In Esperanto, just like in English, you do not need to repeat the object if it clearly belongs to both verbs.

So this structure means:

• Extend the knee and then flex the knee

but saying la genuon once is enough.

A more expanded version could be:

• Etendu la genuon kaj poste fleksu ĝin.

That is also correct, but the original version is shorter and very natural.

Yes, a little.

Esperanto word order is fairly flexible, as long as the grammar stays clear. For example, you could also say:

• Etendu la genuon kaj poste fleksu ĝin.
• Poste fleksu la genuon.

But the original sentence is compact and natural for an instruction. The -n on genuon helps keep the object clear even if the word order changes.

It is pronounced in separate syllables, roughly:

• ge-nu-on

The stress in Esperanto always falls on the second-to-last syllable, so here the stress is on nu:

• ge-NU-on

That regular stress rule is very helpful in Esperanto, because it almost never changes.