Mia fratino diris, ke ŝi estos fininta la tradukon antaŭ dimanĉo, se ŝi havos trankvilan vesperon morgaŭ.

Ke introduces a subordinate clause and usually corresponds to English that.

So:

• Mia fratino diris = My sister said
• ke ŝi estos fininta la tradukon... = that she will have finished the translation...

Esperanto uses ke very regularly after verbs like diri (say), pensi (think), scii (know), and so on.

Because Esperanto commonly separates subordinate clauses with commas.

In this sentence:

• Mia fratino diris, ke...
• ..., se ŝi havos...

That punctuation is very normal in Esperanto. It is often more consistent than in English, where commas before that are less common.

Because ŝi is the normal subject pronoun meaning she, and si is a reflexive form that is not used as a subject.

Important point:

• ŝi = she
• si / sia = reflexive object or reflexive possessive, referring back to the subject of the same clause

So here the subject of the subordinate clause must be ŝi:

• ke ŝi estos fininta...

You could use sia later in the same clause if you wanted a reflexive possessive, for example sian tradukon = her own translation.

Estos fininta is a compound form meaning will have finished. It emphasizes that the action will already be completed before a certain future point.

So:

• ŝi finos la tradukon antaŭ dimanĉo = she will finish the translation before Sunday
• ŝi estos fininta la tradukon antaŭ dimanĉo = she will have finished the translation before Sunday

Both are possible, but estos fininta highlights the completed result by that deadline.

Esperanto often prefers simpler tense forms when the meaning is clear, but the compound form is perfectly grammatical and useful when you want this exact nuance.

Fininta is the active past participle of fini.

It breaks down like this:

• fin- = finish
• -int- = active past participle, meaning having done
• -a = adjective ending

So fininta literally means having finished.

With esti, it forms a compound tense:

• estas fininta = has finished / is in the state of having finished
• estis fininta = had finished
• estos fininta = will have finished

Because Esperanto does not have the same mandatory tense backshifting that English often has in reported speech.

English often changes:

• She said: I will have finished
• reported as She said that she would have finished

Esperanto usually keeps the tense that directly expresses the time relationship:

• Ŝi diris, ke ŝi estos fininta...

That is normal Esperanto.

Using estus fininta would add a conditional or hypothetical meaning, not just ordinary reported speech. So it would not be the neutral equivalent here.

Because this is a real future condition.

• se ŝi havos trankvilan vesperon morgaŭ = if she has a quiet evening tomorrow

In English, we usually say if she has, not if she will have, but Esperanto does not follow that rule. Using the future tense after se is normal when the condition is clearly in the future.

By contrast:

• havus would sound hypothetical or less real, like if she had
• havos fits the straightforward future situation here

Because they are direct objects, so they take the accusative ending -n.

• fini la tradukon = to finish the translation
• havi trankvilan vesperon = to have a quiet evening

Also, adjectives agree with their nouns in Esperanto, so if the noun has -n, the adjective gets -n too:

• trankvila vespero
• trankvilan vesperon

That is why both words in trankvilan vesperon carry the accusative marking.

Because antaŭ is a preposition, and after a preposition Esperanto normally uses the basic noun form, not the accusative.

So:

• antaŭ dimanĉo = before Sunday

The form dimanĉon can be used by itself as a time expression meaning on Sunday, but after antaŭ, you normally say antaŭ dimanĉo.

Because days of the week are often used without the article in ordinary time expressions.

So:

• dimanĉe = on Sunday
• antaŭ dimanĉo = before Sunday

If you said la dimanĉo, that would point to a more specifically identified Sunday in the context. Without la, the phrase is more general and natural here.

Literally, it means before Sunday.

In context, English might sometimes translate that idea as by Sunday, because for deadlines English often uses by. But the Esperanto phrase itself points to a time earlier than Sunday.

If you wanted to be especially clear about including Sunday as the final limit, you might choose a different wording, such as:

• ĝis dimanĉo = until Sunday
• plej malfrue dimanĉe = at the latest on Sunday
• antaŭ la fino de dimanĉo = before the end of Sunday

So antaŭ dimanĉo is best understood as before Sunday begins.

Because la tradukon simply means the translation, referring to a specific translation known from context.

If you said sian tradukon, that would mean her own translation and would explicitly show reflexive possession.

So the difference is:

• la tradukon = the translation
• sian tradukon = her own translation

The sentence does not need to mark ownership, so la tradukon is fine.

Here morgaŭ most naturally goes with the condition:

• se ŝi havos trankvilan vesperon morgaŭ = if she has a quiet evening tomorrow

Yes, it can be moved, because Esperanto word order is fairly flexible. For example:

• se ŝi morgaŭ havos trankvilan vesperon
• morgaŭ, se ŝi havos trankvilan vesperon...

The original placement is completely natural. Putting morgaŭ at the end is just one normal way to do it.