Antaŭ la vespero ŝi estos konfesinta la veron al sia patrino.

It is the Esperanto equivalent of the future perfect.

• estos = future form of esti (will be)
• konfesinta = having confessed

Together, ŝi estos konfesinta means that at some future point, the action of confessing will already be completed.

So the sentence does not just say that she will confess. It says that by that future time, she will already have confessed.

Because participles in Esperanto are adjective-like forms.

konfesinta is built like this:

• konfes- = confess
• -int- = active past participle, meaning having done
• -a = adjective ending

So konfesinta literally means having confessed or one who has confessed.

In compound forms with esti, Esperanto uses this participle form, not an infinitive. Since the subject is singular ŝi, the form is singular too: konfesinta. If the subject were plural, you would get konfesintaj.

Because ŝi konfesos and ŝi estos konfesinta are not quite the same.

• ŝi konfesos la veron = she will confess the truth
• ŝi estos konfesinta la veron = she will have confessed the truth

The simple future says the action will happen in the future.

The future perfect says that before another future reference point, the action will already be finished. Here that reference point is antaŭ la vespero.

It is fully grammatical, but it can sound a bit more formal or precise than everyday speech.

In casual conversation, many speakers might choose a simpler sentence if the meaning is obvious from context. But if you specifically want to express completion before a future moment, estos konfesinta is a very clear and correct choice.

So this is a good form to know, even if simpler wording is often preferred in informal use.

Literally, it means before the evening.

In natural English, because the sentence uses a future-perfect idea, you would often translate it as by evening. That is because English often uses by + time where Esperanto uses antaŭ + time expression in this kind of context.

So the idea is that the confession will be completed before evening arrives.

Here la vespero refers to a specific, understood evening: the evening relevant to the situation.

Esperanto often uses la with times or parts of the day when a definite occasion is meant. So antaŭ la vespero is not talking about evenings in general, but about that evening / the coming evening.

Because la veron is the direct object of konfesi.

She is confessing what?
→ la veron

That is why it takes -n:

• la vero = the truth
• la veron = the truth, as a direct object

Because it means the truth, not just truth in a general abstract sense.

Here we are talking about a specific truth that she is confessing, so Esperanto uses la just as English does in the truth.

Because al marks the person to whom something is said, confessed, given, and so on.

With konfesi, the usual pattern is:

• konfesi ion al iu = to confess something to someone

So:

• la veron = what is confessed
• al sia patrino = to whom it is confessed

Because sia is the reflexive possessive. It refers back to the subject of the clause.

Here the subject is ŝi, so:

• sia patrino = her own mother (the mother of the subject)
• ŝia patrino would normally mean some other woman’s mother, not the subject’s own mother

This is one of the most important differences for English speakers to learn, because English uses her for both meanings, while Esperanto separates them:

• sia = belonging to the subject
• ŝia = belonging to some female person, not reflexively tied to the subject

Yes. Esperanto word order is fairly flexible.

You could also say:

Ŝi estos konfesinta la veron al sia patrino antaŭ la vespero.

That is still grammatical and means the same thing.

Putting Antaŭ la vespero first gives special emphasis to the time frame. It tells the listener right away by when the action will be completed.