Se vi sendos al mi mesaĝon, mi tuj respondos al vi.

In Esperanto, -os is the future-tense ending. So sendos means will send.

The verb here comes from sendi = to send:

• mi sendas = I send / am sending
• mi sendis = I sent
• mi sendos = I will send

So vi sendos means you will send.

Because the whole sentence talks about a future situation:

• Se vi sendos al mi mesaĝon = If you send / will send me a message
• mi tuj respondos al vi = I will reply to you immediately

Esperanto often uses the future in both parts when both actions are clearly future. English sometimes says If you send me a message, I’ll reply, but Esperanto can naturally say Se vi sendos..., mi respondos...

Yes, that is also possible, depending on the nuance.

• Se vi sendas... can sound more general or more like if/when you send...
• Se vi sendos... focuses more clearly on a future event

Both are understandable and acceptable in many contexts. The version with -os in both clauses is very straightforward for a learner because it clearly marks the future in both actions.

Because sendi and respondi here need the preposition al to show the direction or recipient:

• sendi ion al iu = to send something to someone
• respondi al iu = to reply to someone

So:

• al mi = to me
• al vi = to you

This is different from English, where me and you can appear without to in some sentences, such as send me a message. Esperanto often keeps the relationship explicit with al.

The -n marks the direct object.

Here, the thing being sent is a message, so mesaĝo becomes mesaĝon.

Compare:

• Mi sendas mesaĝon. = I send a message.
• Mesaĝo venis. = A message arrived.

In the sentence:

• vi sendos al mi mesaĝon

the message is what is being sent, so it takes the accusative -n.

Because they are not direct objects in this sentence.

• mi in al mi is part of a prepositional phrase: to me
• vi in al vi is also part of a prepositional phrase: to you

After a preposition like al, you normally do not add -n just because it is the receiver of the action.

So:

• al mi = to me
• al vi = to you

But:

• Mi vidas vin. = I see you.

Here vin has -n because it is the direct object.

Tuj means immediately, right away, or at once.

In the sentence:

• mi tuj respondos al vi

it modifies respondos: I will reply immediately.

Esperanto word order is fairly flexible, so you may also see:

• Mi respondos tuj al vi.
• Tuj mi respondos al vi.

But mi tuj respondos al vi is very natural and clear.

Not completely. Esperanto word order is more flexible than English because endings show grammatical roles.

This sentence is natural as:

• Se vi sendos al mi mesaĝon, mi tuj respondos al vi.

But some parts could move:

• Se vi al mi sendos mesaĝon, mi tuj respondos al vi.
• Mi respondos al vi tuj.

Still, the original order is the most neutral and easiest for learners:

• subject + verb + prepositional phrase + object
• then subject + adverb + verb + prepositional phrase

The comma separates the if-clause from the main clause.

• Se vi sendos al mi mesaĝon, = If you send me a message,
• mi tuj respondos al vi. = I will reply to you immediately.

This is very standard and helps readability. In Esperanto, commas are commonly used to separate this kind of subordinate clause from the main clause.

Vi can mean:

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

Esperanto uses the same pronoun for all of these.

So this sentence could mean:

• If you (one person) send me a message...
• If you all send me a message...

The context tells you which is meant.

In Esperanto, the dictionary form of a verb ends in -i. That is the infinitive:

• sendi = to send
• respondi = to reply

To make a finite verb, you replace -i with a tense ending:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = imperative/jussive

So:

• sendi → sendos
• respondi → respondos

Yes, mesaĝo is a normal word meaning message.

It is pronounced roughly:

• meh-SAH-joh

A few pronunciation notes:

• ĝ sounds like the g in George or judge
• j in Esperanto is like English y
• stress falls on the second-to-last syllable: me-SA-ĝo

So mesaĝo has stress on sa.

Sometimes context allows omission, but in this sentence both al mi and al vi are useful and natural.

• Se vi sendos mesaĝon, mi tuj respondos. = If you send a message, I’ll reply immediately.

This is grammatical, but it no longer explicitly says to me and to you. The original sentence is clearer and more complete:

• Se vi sendos al mi mesaĝon, mi tuj respondos al vi.

In Esperanto, the normal pattern is:

• respondi al iu = to reply to someone
• respondi ion = to answer something

So you can say:

• Mi respondis al ŝi. = I replied to her.
• Mi respondis la demandon. = I answered the question.

In your sentence, the person receiving the reply is vi, so Esperanto uses al vi:

• mi tuj respondos al vi

Yes. You could say:

• Se vi sendos mesaĝon al mi, mi tuj respondos al vi.

That is also correct and very natural. Both orders are fine:

• sendos al mi mesaĝon
• sendos mesaĝon al mi

The original version slightly emphasizes the recipient earlier, but the difference is small. Both are standard Esperanto.