Postmorgaŭ mi respondos al ŝi, kiam mi finos la raporton por mia estro.

Yes. Postmorgaŭ is formed from:

• post = after
• morgaŭ = tomorrow

Together, it means the day after tomorrow.

Esperanto often builds words very logically like this. Even though it is made from two familiar parts, postmorgaŭ functions as a single time word in the sentence.

Because -os is the Esperanto future-tense ending.

• respondas = answers / is answering
• respondis = answered
• respondos = will answer

Since the action happens postmorgaŭ, the future tense is the natural choice: mi respondos = I will answer.

Because with the person being answered, Esperanto normally uses al.

So:

• respondi al iu = to answer someone
• ŝi = she
• al ŝi = to her

This is different from English, where answer someone does not need a preposition. In Esperanto, the person receiving the answer is usually introduced by al.

No. Si is a reflexive pronoun, and it refers back to the subject of the same clause.

Here the subject of the main clause is mi. So:

• mi respondos al ŝi = I will answer her

If you used si, it would have to refer back to mi, which does not make sense here, because si is not used for I or you in this way.

So ŝi is correct.

In Esperanto, subordinate clauses often use the tense that matches the real time of the action.

Both actions are in the future:

• mi respondos = I will answer
• mi finos = I will finish

So kiam mi finos is very natural: the finishing is also future from the speaker’s point of view.

This is one place where Esperanto differs from English. English often says when I finish even though it refers to the future. Esperanto more straightforwardly uses the future tense: kiam mi finos.

Because la raporton is the direct object of finos.

The speaker is finishing the report, so raporto takes the accusative ending -n:

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

This -n ending is one of the most important features of Esperanto grammar.

Because por means for, in the sense of intended for, on behalf of, or for the benefit of.

• por mia estro = for my boss

If you said al mia estro, that would mean to my boss, emphasizing direction toward the boss, as if something were being sent or given to him or her.

Here the idea is that the report is being prepared for the boss, so por is the better choice.

Because possessive words like mia, via, lia, ŝia, and so on usually take the place of the article.

So Esperanto normally says:

• mia estro = my boss
• not la mia estro

Using la with a possessive is only done in special cases, and not here.

Esperanto word order is fairly flexible, because endings like -n and prepositions like al and por already show the grammatical roles.

So these are also possible:

• Mi respondos al ŝi postmorgaŭ, kiam mi finos la raporton por mia estro.
• Kiam mi finos la raporton por mia estro, mi respondos al ŝi postmorgaŭ.

The original order is perfectly natural, but it is not the only possible order.

Because kiam mi finos la raporton por mia estro is a subordinate clause.

Esperanto commonly uses commas to separate main clauses from subordinate clauses, especially when words like kiam, se, ĉar, and kvankam introduce them.

So the comma helps show the structure:

• main clause: Postmorgaŭ mi respondos al ŝi
• subordinate clause: kiam mi finos la raporton por mia estro

Yes. Time expressions are often movable in Esperanto.

These are all possible, with slight differences in emphasis:

• Postmorgaŭ mi respondos al ŝi...
• Mi respondos al ŝi postmorgaŭ...
• Mi postmorgaŭ respondos al ŝi... (less common, but possible)

Putting postmorgaŭ first gives it a bit more emphasis. It highlights the time right away.

A few pronunciation points:

• ŝ sounds like English sh
• aŭ sounds roughly like ow in cow
• stress in Esperanto usually falls on the second-to-last syllable

So:

• ŝi sounds like she
• morgaŭ is roughly MOR-gow
• postmorgaŭ is roughly post-mor-GOW

The spelling is very regular, so once you know the sound values, pronunciation is usually straightforward.