Ŝi fleksos la genuon, krom se la artiko doloros.

Because la genuon is the direct object of fleksos. In Esperanto, the direct object takes the accusative ending -n.

• Ŝi fleksos la genuon = She will bend the knee.
• The thing being bent is the knee, so it gets -n.

The basic noun is genuo. With the accusative ending, it becomes genuon.

Because la artiko is the subject of doloros, not the object.

In la artiko doloros:

• la artiko = the joint
• doloros = will hurt / will ache

So the joint is the thing that hurts. Subjects do not take -n.

Compare:

• La artiko doloros. = The joint will hurt.
• Ŝi fleksos la artikon. = She will bend the joint.

In the second sentence, artikon would take -n because it is the object.

Krom se means unless.

Literally, krom means apart from / except, and se means if. Together they form the expression unless.

So:

• Ŝi fleksos la genuon, krom se la artiko doloros.
• = She will bend the knee, unless the joint hurts.

It introduces an exception to what was just said.

Because the sentence is talking about a future situation.

• fleksos = will bend
• doloros = will hurt

In Esperanto, it is normal to use the future tense in both parts if both actions are understood as future.

English often says unless the joint hurts, using the present tense after unless, but Esperanto can simply use the future: krom se la artiko doloros.

Sometimes, yes, depending on context, but doloros is very straightforward here because it clearly points to a future possibility.

• krom se la artiko doloros = unless the joint will hurt / unless the joint hurts then
• krom se la artiko doloras could sound more like a present, current condition: unless the joint is hurting

So doloros fits well if the speaker means a future reaction or future condition.

Because the sentence refers to a specific knee and a specific joint that are understood from context.

In Esperanto, la is used when the listener can identify what is meant. With body parts, this often happens naturally, especially in medical, physical, or instructional contexts.

So:

• la genuon = the knee
• la artiko = the joint

This does not necessarily mean the in a strongly emphatic English sense; it often just means the relevant, known body part.

Genuo means knee.
Artiko means joint.

A genuo is a specific body part. An artiko is a more general anatomical term for a place where bones meet and move.

In this sentence, la artiko probably refers to the knee joint, but the speaker uses the more general word artiko in the second clause.

No object is missing. Dolori is an intransitive verb here, meaning to hurt, to ache, or to be painful.

So:

• La artiko doloras. = The joint hurts.
• La artiko doloros. = The joint will hurt.

The joint itself is the thing experiencing pain, so no direct object is needed.

Fleksos is made of:

• fleks- = bend / flex
• -os = future tense ending

So fleksos means will bend.

This is a very regular Esperanto pattern:

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

Usually, yes. Esperanto normally states the subject pronoun because the verb ending shows tense, but not person.

For example:

• fleksos could mean I will bend, you will bend, she will bend, they will bend, etc., depending on context.

So ŝi is useful and normally expected to make the sentence clear:

• Ŝi fleksos... = She will bend...

Because the sentence contains two clauses:

1. Ŝi fleksos la genuon
2. krom se la artiko doloros

The comma helps separate the main statement from the exception clause. This is very common and natural in Esperanto punctuation.