La helpon promesinta amikino alvenis frue.

Promesinta is a past active participle used like an adjective.

It is built from:

• promes- = the root promise
• -int- = having done / who did earlier
• -a = adjective ending

So promesinta means something like:

• having promised
• who had promised

In this sentence, helpon promesinta amikino means a female friend who had promised help.

Because helpon is the direct object of promesi.

The friend is the one who did the promising, and help is what was promised. In Esperanto, direct objects usually take -n.

So:

• promesi helpon = to promise help
• helpon promesinta = having promised help

Even though promesinta is a participle and not a full finite verb, it still keeps its object just like the verb does.

Because amikino is the subject of the main verb alvenis.

The sentence is about the female friend arriving early. So:

• amikino = the one who arrived
• helpon = the thing that was promised

Esperanto marks this clearly with -n on the object, so you can tell the roles apart.

Esperanto often uses a participle phrase instead of a relative clause.

English might say:

• The female friend who had promised help arrived early.

Esperanto can say the same idea more compactly:

• La helpon promesinta amikino alvenis frue.

So helpon promesinta works like who had promised help.

A fuller version with a relative clause would be:

• La amikino, kiu promesis helpon, alvenis frue.

Both are possible, but the participle version is shorter and very natural.

Because it is acting like an adjective that describes amikino.

In Esperanto, adjectives can come before or after the noun. So this sentence could also be written as:

• La amikino promesinta helpon alvenis frue.

That still means the same thing.

The version with helpon promesinta before the noun is a compact way to keep the whole descriptive phrase together before amikino.

This is about time relative to the main action.

• promesanta = promising, who is promising
• promesinta = having promised, who had promised earlier

So:

• La helpon promesanta amikino...
  = the female friend who is promising help

• La helpon promesinta amikino...
  = the female friend who had promised help

In your sentence, promesinta shows that the promising happened before the arrival.

It applies to the whole noun phrase.

So La helpon promesinta amikino means:

• the female friend who had promised help

The word la marks the whole person being talked about as definite, not just the noun by itself.

Because -in- marks a female being.

• amiko = friend
• amikino = female friend

So the sentence specifically refers to a woman or girl.

If the person’s sex were not being specified, some speakers might use amiko, depending on context and style.

Alvenis is the past tense of alveni (to arrive).

• alvenas = arrives / is arriving
• alvenis = arrived

So the sentence places the arrival in the past.

The participle promesinta does not itself replace the main verb tense. It just tells you that the promising happened earlier than the main action.

Yes, it functions as one descriptive unit modifying amikino.

You can think of it as:

• helpon = object inside the phrase
• promesinta = participle head of the phrase

Together they describe the friend:

• the help-promising / help-having-promised female friend

More natural English would be the female friend who had promised help, but grammatically Esperanto packs that idea into a compact modifier.

Yes. A beginner-friendly version would be:

• La amikino, kiu promesis helpon, alvenis frue.

This uses:

• kiu = who
• promesis = promised

It means the same thing, but many learners find it easier to understand at first than the participle version.

The original sentence is just a more condensed way to say it.