Kiam vi alvenos, mi jam estos fininta la raporton kaj sendinta ĝin al la estro.

Because the sentence refers to a real event in the future: when you will arrive. In Esperanto, it is normal to use the future tense in a kiam clause if the action really is future.

So:

• Kiam vi alvenos = When you arrive / when you will arrive
• mi jam estos fininta... = I will already have finished...

English often uses the present tense after when (When you arrive), but Esperanto can use the future directly here.

Estos fininta is a future form meaning will have finished.

It is made from:

• estos = will be
• fininta = having finished / finished (active past participle)

Together, esti + active past participle expresses that by some future point, the action will already be completed.

So:

• mi estos fininta la raporton = I will have finished the report

This is similar to the English future perfect.

Esperanto does not have a single special verb ending for the English-style future perfect. Instead, it builds that meaning analytically:

• esti in the future: estos
• plus an active past participle: fininta

So the language says, in effect, I will be in the state of having finished.

This is a normal Esperanto way to express will have done.

Sendinta works the same way as fininta. It means having sent.

So:

• mi jam estos fininta la raporton kaj sendinta ĝin al la estro

means:

• I will already have finished the report and sent it to the boss

The verb estos is understood for both participles:

• estos fininta
• (estos) sendinta

Esperanto often avoids repeating a word when it is clearly shared.

Because here they are part of the predicate with estos.

Compare:

• Mi estas fininta. = I have finished / I am in the state of having finished.
• Fininte la raporton, mi iris hejmen. = Having finished the report, I went home.

So:

• -inta = participle used adjectivally or in compound verb-like structures with esti
• -inte = adverbial participle, meaning having done...

In your sentence, the participles belong with estos, so fininta and sendinta are correct.

Because it is the direct object of fininta / fini.

• la raporto = the report
• la raporton = the report as the object

In Esperanto, direct objects normally take -n.

So:

• mi finos la raporton
• mi estas fininta la raporton
• mi estos fininta la raporton

all use la raporton because the report is what is being finished.

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

• la raporton = the report
• ĝin = it

It also has -n because it is the direct object of sendinta / sendi.

So:

• sendinta ĝin = having sent it
• it = the report

Using ĝin avoids repeating la raporton.

Because sendi ion al iu means to send something to someone.

Here:

• ĝin = the thing being sent
• al la estro = to the boss

The preposition al already marks the recipient, so there is no need for -n on estro.

So:

• sendi la raporton al la estro = to send the report to the boss

Jam means already.

It emphasizes that by the time the other future action happens, the speaker’s actions will be completed.

So the timeline is:

1. You arrive.
2. By that moment, I already will have finished the report.
3. By that moment, I already will have sent it to the boss.

Without jam, the sentence would still be grammatical, but jam makes the completed-before-that-time idea stronger and more natural.

Not naturally in English. A very literal breakdown can help you see the Esperanto structure, but the best English translation is:

• When you arrive, I will already have finished the report and sent it to the boss.

The Esperanto structure uses estos + participle, but the natural English equivalent is the future perfect: will have finished, will have sent.

In meaning, it is usually best translated as when you arrive.

Even though Esperanto uses the future form alvenos, English normally uses the present after when for future time clauses.

So:

• Kiam vi alvenos = natural English: When you arrive
• not usually: When you will arrive

Both refer to the future, but the natural grammar is different in the two languages.

Yes. In everyday speech, many speakers might choose a simpler structure if the context is clear. For example:

• Kiam vi alvenos, mi jam finos la raporton kaj sendos ĝin al la estro.

But this can also mean that those actions happen around that future time, not necessarily that they are completed before your arrival.

The original sentence is more precise:

• mi jam estos fininta... kaj sendinta...

It clearly says both actions will already be completed by the time you arrive.

Because both participles depend on the same auxiliary verb.

The full expanded structure would be:

• mi jam estos fininta la raporton kaj estos sendinta ĝin al la estro

But repeating estos is unnecessary and sounds clumsy. Esperanto commonly uses one auxiliary for coordinated participles when the meaning is clear:

• estos fininta ... kaj sendinta ...

This is just like English:

• I will have finished the report and sent it to the boss

You do not need to repeat will have before sent either.

Not here in any visible way, because mi does not show gender or number agreement on the participle.

But participles used adjectivally can take -j and -n when needed, just like adjectives.

Here, though, fininta and sendinta are predicate participles with estos, so they stay in the basic form:

• mi estos fininta
• ni estos finintaj would be possible if you explicitly wanted plural agreement with ni

In practice, learners mainly need to recognize that fininta and sendinta are describing the subject as someone who will by then have completed those actions.

Kiam means when and introduces a time clause.

In this sentence, it sets the reference point for the future perfect idea:

• Kiam vi alvenos = the future moment
• mi jam estos fininta... = what will be completed by that moment

So the whole sentence is organized around a timeline:

• your arrival = time reference
• my finishing and sending = already completed before that reference point