La mesaĝon sendinta amikino atendas mian respondon.

Sendinta is the active past participle of sendi = to send.

It breaks down like this:

• send- = the root send
• -int- = active past participle, meaning having done the action
• -a = adjective ending

So sendinta means having sent or who sent.

In this sentence, la mesaĝon sendinta amikino means the female friend who sent the message.

Because mesaĝon is not the object of atendas. It is the object of sendinta.

Think of the sentence in two parts:

• La mesaĝon sendinta amikino = the friend who sent the message
• atendas mian respondon = is waiting for my reply

So mesaĝon gets -n because it is what was sent.

A useful way to see it is this:

• La amikino sendis la mesaĝon.
• La amikino atendas mian respondon.

These ideas are combined into one sentence, and la mesaĝon keeps its role as the object of sendi.

Because amikino is the subject of atendas.

The friend is the one doing the waiting, so amikino stays in the nominative form:

• amikino = subject
• mian respondon = object of atendas

So the basic structure is:

• [The friend who sent the message] = subject
• waits
• [my reply] = object

Esperanto often uses a participle where English uses a relative clause.

English:

• the friend who sent the message

Esperanto:

• la mesaĝon sendinta amikino

This is a more compact way of saying the same thing.

If you want, you can also say it with a relative clause:

• La amikino, kiu sendis la mesaĝon, atendas mian respondon.

That means the same thing. The participial version is simply more condensed.

Because sendinta is an adjective, and here it belongs to the noun phrase centered on amikino.

In Esperanto, adjectives can describe nouns before or after them, and participles used with -a behave like adjectives.

So:

• sendinta amikino = a friend who has sent
• la mesaĝon sendinta amikino = the friend who sent the message

The participle agrees with the noun it describes. Here both are:

• singular
• nominative

So we get:

• sendinta
• amikino

If it were plural, the participle would also become plural:

• la mesaĝon sendintaj amikinoj

Because la applies to the whole noun phrase:

• la mesaĝon sendinta amikino

This entire group means the female friend who sent the message.

Esperanto usually does not repeat la inside the same noun phrase unless there is a separate reason to do so. One la is enough to mark the whole phrase as definite.

Because adjectives agree with the nouns they describe.

• respondo = reply
• respondon = reply as a direct object
• mia = my
• mian = my, agreeing with an accusative noun

So:

• mian respondon = my reply as the object of atendas

In Esperanto, if the noun takes -n, its adjective or possessive adjective also takes -n.

This is an important participle contrast:

• sendanta = sending, who is sending
• sendinta = having sent, who sent

So:

• la mesaĝon sendanta amikino = the friend who is sending the message
• la mesaĝon sendinta amikino = the friend who sent the message

In your sentence, sendinta shows that the sending happened before the waiting.

Yes. That is the idea here.

The friend first sent the message, and now she is waiting for the reply.

That is exactly why -int- is used: it marks an action as already completed relative to the main situation.

So the timeline is:

1. she sent the message
2. she is waiting for my reply

Yes. A learner-friendly version would be:

• La amikino, kiu sendis la mesaĝon, atendas mian respondon.

That uses a relative clause and may feel more natural at first if you are coming from English.

The original sentence:

• La mesaĝon sendinta amikino atendas mian respondon.

is more compact and stylistically neat, but the meaning is the same.

Because -in- marks the feminine form.

• amiko = friend
• amikino = female friend

So the sentence specifically says the friend is female.

If the speaker did not want to specify that, they might use another wording depending on style and context, but in this sentence amikino clearly means female friend.

Yes, to some extent.

Esperanto word order is fairly flexible, as long as the grammar stays clear. For example, these are possible ways to express the same idea:

• La mesaĝon sendinta amikino atendas mian respondon.
• La amikino sendinta la mesaĝon atendas mian respondon.
• La amikino, kiu sendis la mesaĝon, atendas mian respondon.

The original version puts la mesaĝon before sendinta, which is common in compact participial expressions. But other wordings may feel easier to parse for learners.

Not exactly. It comes from a verb, but here it is functioning as an adjective.

That is because it ends in -a.

Compare:

• sendis = sent → finite verb
• sendinta = having sent / who sent → participial adjective

So in this sentence, sendinta is not the main verb. The main verb is:

• atendas = waits / is waiting

The participle just adds information about amikino.