Anstataŭ ke ŝi streĉos la ĉenon, mi riparos la dentradon.

Anstataŭ ke introduces a subordinate clause meaning instead of / rather than. It sets up one possible action as the alternative, and then the main clause states what will happen instead.

So in this sentence:

• Anstataŭ ke ŝi streĉos la ĉenon = the alternative action
• mi riparos la dentradon = what will actually happen instead

Because the sentence uses a full clause after anstataŭ:

• ŝi = subject
• streĉos = finite verb

When Esperanto links in a full clause like that, ke is the normal connector.

Compare:

• anstataŭ la riparo = instead of the repair
• anstataŭ ripari = instead of repairing
• anstataŭ ke ŝi streĉos la ĉenon = instead of her tightening the chain

So ke is needed because what follows is not just a noun or infinitive, but a whole clause.

Because the subordinate clause has its own verb, and that verb can show its own tense normally. Here -os marks future:

• streĉos = will tighten
• riparos = will repair

An English speaker may expect something more like instead of tightening, but Esperanto does not have to copy that structure. With anstataŭ ke, you can use a normal finite verb.

So the future tense here shows that the tightening is a future possibility or plan that is being replaced by the repairing.

Not naturally in this exact sentence, because streĉi is an infinitive and does not show its own subject. In Esperanto, an infinitive phrase usually connects most naturally to the same subject as the main clause.

For example:

• Anstataŭ streĉi la ĉenon, mi riparos la dentradon.

This normally suggests that mi is the one who would have tightened the chain.

But in your sentence, the one who would tighten the chain is ŝi, not mi. So a full clause with ke ŝi ... makes the subject clear.

Because ŝi is the subject of streĉos.

In Esperanto:

• subjects do not take -n
• direct objects do take -n

Here:

• ŝi streĉos = she will tighten → ŝi is the subject
• la ĉenon = the thing being tightened → direct object
• la dentradon = the thing being repaired → direct object

So ŝi is correct.

Because they are direct objects.

• streĉi la ĉenon = to tighten the chain
• ripari la dentradon = to repair the gear/cogwheel

The -n ending marks the noun that receives the action of the verb.

This is one of the most important Esperanto patterns:

• mi vidas la hundon = I see the dog
• ŝi streĉos la ĉenon = she will tighten the chain
• mi riparos la dentradon = I will repair the gear

It is a compound word:

• dent- = tooth
• rad- = wheel
• -o = noun ending

So dentrado literally means toothed wheel, which is why it means a gear or cogwheel.

This kind of word-building is very common in Esperanto, and it is often helpful to break long words into parts.

Because the sentence is talking about specific things:

• la ĉenon = the chain
• la dentradon = the gear/cogwheel

Esperanto la works much like English the. It usually means the speaker expects the listener to know which chain and which gear are meant from the situation or earlier context.

Without la, the nouns would sound more indefinite or generic.

Because Anstataŭ ke ŝi streĉos la ĉenon is a subordinate clause placed before the main clause. Esperanto commonly separates such clauses with a comma.

So the structure is:

• subordinate clause
• comma
• main clause

That is standard and helps readability.

Yes. Esperanto word order is fairly flexible here. You could also put the main clause first:

• Mi riparos la dentradon, anstataŭ ke ŝi streĉos la ĉenon.

The basic meaning stays the same. Putting the anstataŭ ke clause first gives the contrast earlier and makes the sentence feel a bit more front-loaded.

No. In real Esperanto, you may also come across other choices depending on nuance and style. The sentence as given uses a normal future form, which is understandable because the alternative action is in the future.

But the main point for a learner is this:

• after anstataŭ ke, Esperanto can use a full subordinate clause
• that clause has its own subject and verb
• the verb form depends on the meaning the speaker wants

So the important thing to notice here is the clause structure, not just the single ending.

A few letters here are especially worth noticing:

• ŝ sounds like English sh
• ĉ sounds like English ch

So:

• ŝi sounds like shee
• ĉenon begins with a ch sound

This is a very common early question for English-speaking learners, because Esperanto spelling is very regular once you learn the special letters.