Mi sendas al vi mesaĝon matene.

Because mesaĝon is the direct object of the verb sendas. In Esperanto, direct objects usually take the accusative ending -n.

• Mi sendas mesaĝon = I send a message
• mesaĝo = a message
• mesaĝon = a message, as the thing being sent

So the -n shows what is receiving the action.

Because vi here is an indirect object, not a direct object.

• mesaĝon = the thing being sent
• al vi = to you, the receiver

Esperanto often uses al for to when showing the person something is given or sent to.

Compare:

• Mi vidas vin = I see you
  Here vin is the direct object, so it gets -n.
• Mi sendas mesaĝon al vi = I send a message to you
  Here vi is after al, so it does not take -n.

Al usually means to. In this sentence, it marks the direction or recipient of the action.

So:

• Mi sendas = I send
• al vi = to you

This is very similar to English send to you.

Because matene is an adverb, meaning in the morning or during the morning.

The ending -e makes it adverbial.

• mateno = morning
• matene = in the morning / morning-time

Esperanto often uses a bare adverb like this for times:

• tage = by day / during the day
• nokte = at night
• vespere = in the evening
• matene = in the morning

Yes, you could, but matene is shorter and more natural in many cases.

• matene = in the morning
• en la mateno = in the morning

Both are understandable, but Esperanto often prefers the simple adverb form when talking about time expressions like this.

It can mean either, depending on context.

The Esperanto present tense -as covers both simple present and present progressive meanings that English separates:

• Mi sendas = I send / I am sending

So this sentence could mean:

• I send you a message in the morning.
• I am sending you a message this morning.

The surrounding context tells you which is intended.

By itself, it could be either.

Esperanto present tense does not force the same distinction English often does.

So Mi sendas al vi mesaĝon matene might mean:

• a habit: I send you a message in the morning
• a current action in a morning context: I am sending you a message in the morning

Usually the situation, extra words, or previous sentences make it clear.

Yes. Esperanto word order is fairly flexible, because endings show grammatical roles.

This sentence could also appear as:

• Matene mi sendas al vi mesaĝon.
• Mesaĝon mi sendas al vi matene.
• Al vi mi sendas mesaĝon matene.

All of these are grammatical. The most neutral order is the original one:

Mi sendas al vi mesaĝon matene.

Changing the order usually changes emphasis, not the core meaning.

Esperanto has no indefinite article. English uses a/an, but Esperanto does not.

So:

• mesaĝo can mean a message
• mesaĝon can mean a message as a direct object

Esperanto does have a definite article:

• la mesaĝo = the message
• la mesaĝon = the message, as a direct object

Since the sentence just says mesaĝon, it means a message or simply message, depending on context.

It can be either.

Vi is used for:

• you singular
• you plural

It is also used for both informal and formal you. Esperanto does not normally distinguish those in everyday usage.

So al vi could mean to you for one person or for several people, depending on context.

No, that would be wrong.

Mi sendas vin mesaĝon would treat vin as a direct object, which would suggest you are the thing being sent, not the recipient.

In this sentence:

• the thing sent = mesaĝon
• the recipient = al vi

So al is necessary here.

Here is the breakdown:

• Mi = pronoun, I
• sendas = verb in the present tense, send
• al = preposition, to
• vi = pronoun, you
• mesaĝon = noun with accusative -n, message as direct object
• matene = adverb, in the morning

This is a very typical Esperanto sentence structure: subject + verb + indirect object phrase + direct object + time adverb.

Yes, very naturally.

You can say:

Matene mi sendas al vi mesaĝon.

That puts a little more focus on the time. In Esperanto, time expressions are often placed at the beginning, just as in English.

Both versions are fine:

• Mi sendas al vi mesaĝon matene.
• Matene mi sendas al vi mesaĝon.

No difference in basic meaning, only in grammatical function.

• mesaĝo = message
• mesaĝon = message as the direct object

The root meaning stays the same. The -n just shows the role the noun has in the sentence.