Se vi sendos al mi mesaĝon, kiam vi alvenos, mi tuj scios, ke via vojaĝo estis bona.

Esperanto does not follow the same rule as English here. If the action is in the future, Esperanto often uses the future tense normally, even inside a se-clause.

So Se vi sendos... is perfectly natural for a real future condition.

Using sendas would change the feeling. It could suggest something habitual, general, or a more vivid present-style way of speaking, not the straightforward future meaning.

The -n marks the direct object.

Here, the thing being sent is mesaĝo, so it becomes mesaĝon because it is what sendi acts on.

• sendi mesaĝon = to send a message

Because mi is the recipient, not the direct object.

With sendi, Esperanto usually works like this:

• the thing sent = direct object: mesaĝon
• the person receiving it = al
  • person: al mi

So:

• sendi mesaĝon al mi = send a message to me

If you said min, that would make me the direct object, as if I were what was being sent.

It tells us when the message will be sent. In other words, it modifies the idea of sendos al mi mesaĝon.

So the sense is: if you send me a message when you arrive, then I will know...

It is a time clause introduced by kiam.

For the same basic reason as sendos: the arrival is also a future event.

Esperanto usually marks future time directly, even in subordinate clauses like kiam vi alvenos.

So alvenos is natural because the arrival has not happened yet.

Esperanto often uses commas to mark subordinate clauses more regularly than English does.

Here the commas separate the different clause parts:

• Se vi sendos al mi mesaĝon = conditional clause
• kiam vi alvenos = time clause inserted in the middle
• ke via vojaĝo estis bona = content clause after scios

The commas help show the structure clearly.

Tuj means immediately, right away, or at once.

So mi tuj scios means I will know immediately.

Its position is normal. Esperanto adverbs are fairly flexible, but putting tuj before the verb is very common.

Ke introduces a clause that tells what is known.

After verbs like scii, pensi, diri, kredi, Esperanto normally uses ke to introduce the following statement.

So:

• mi scios = I will know
• ke via vojaĝo estis bona = that your trip was good

Because by the time the person arrives and sends the message, the journey is already completed.

So at the moment of knowing, the trip belongs to the past. That is why estis is natural.

Esperanto chooses tense according to the actual time relationship being described, not by copying English tense patterns mechanically.

So the idea is: when I get your message on arrival, I will know that the trip was good.

Because bona describes the noun vojaĝo.

• vojaĝo = a noun
• adjectives describing nouns end in -a
• so: bona vojaĝo

Bone would be an adverb, used to describe a verb, adjective, or another adverb, not a noun.

Either.

Esperanto vi can mean:

• singular you
• plural you
• informal you
• formal you

Context tells you which one is meant.

Normally, no. After scii, if a full statement follows, ke is the standard connector.

So mi tuj scios, ke... is the normal form.

Leaving it out would usually sound incomplete or unnatural in standard Esperanto.

Not here. Se means if in the sense of a condition.

In Esperanto, whether is usually ĉu.

So:

• se = if
• ĉu = whether

This sentence uses se because it gives a condition: if you send the message, then I will know.