Mi sendos la pakon per la poŝto morgaŭ.

Because -os is the Esperanto ending for the future tense.

The verb root is send- = send.
So:

• sendas = send / am sending / is sending
• sendis = sent
• sendos = will send

So Mi sendos means I will send.

A useful point: Esperanto verbs do not change according to the subject. So:

• mi sendos = I will send
• vi sendos = you will send
• li sendos = he will send

The verb form stays the same.

Because la pakon is the direct object of the verb.

In Esperanto, the thing directly affected by the action usually gets the accusative ending -n. Here, the package is the thing being sent, so:

• la pako = the package
• la pakon = the package as a direct object

So in Mi sendos la pakon, the -n shows that the package is what will be sent.

If there were adjectives, they would also take -n:

• la grandan pakon = the big package

Esperanto has only one article: la, which means the.

It does not have a separate word for a/an.

So:

• pako = a package / package
• la pako = the package

That means la pakon refers to a specific package.

Per means by means of, using, or via.

So per la poŝto means by mail or more literally through the postal service.

Some similar examples:

• per aŭto = by car
• per telefono = by telephone
• per interreto = via the internet

So in this sentence, per tells you the method used to send the package.

Because Esperanto and English do not always use articles in the same way.

Here la poŝto can mean the postal service as a whole. So per la poŝto is like saying through the postal service.

Even though English usually says by mail without the, Esperanto can naturally use la here.

In this sentence, poŝto means mail, post, or the postal system.

It does not mean the post office building itself. That would be:

• poŝtejo = post office

Related words:

• poŝto = mail, post, postal service
• letero = letter
• pako = package
• poŝtejo = post office

So per la poŝto means using the mail system.

Yes, often you can.

Poŝte is an adverb meaning by post or by mail. So a sentence like:

• Mi sendos la pakon poŝte morgaŭ

also makes sense.

The difference is mostly one of style:

• per la poŝto = more explicit, literally through the mail/postal service
• poŝte = shorter, more compact

Both are understandable.

Yes, it could go elsewhere.

Morgaŭ is an adverb meaning tomorrow, and Esperanto word order is fairly flexible. For example, these are all possible:

• Mi sendos la pakon per la poŝto morgaŭ.
• Morgaŭ mi sendos la pakon per la poŝto.
• Mi morgaŭ sendos la pakon per la poŝto.

Putting morgaŭ at the end is perfectly normal.

Because Esperanto uses endings like -n to show grammatical roles, word order does not have to carry as much information as it does in English.

Because in Esperanto, many time words work directly as adverbs.

So you say:

• hodiaŭ = today
• hieraŭ = yesterday
• morgaŭ = tomorrow

And you can use them without a preposition:

• Mi venos morgaŭ = I will come tomorrow
• Li alvenis hieraŭ = He arrived yesterday

So morgaŭ by itself is completely normal.

Ŝ is pronounced like English sh.

So poŝto is pronounced roughly like POSH-to.

A couple of useful pronunciation points:

• ŝ = sh
• Esperanto stress normally falls on the second-to-last syllable

So the stress is on POŜ in POŜ-to.