Mi ricevis pakon per la poŝto hodiaŭ matene.

Because pakon is the direct object of ricevis.

In Esperanto, the thing directly affected by the verb usually takes the accusative ending -n.
So:

• Mi ricevis pakon. = I received a package.

Here:

• mi = I
• ricevis = received
• pakon = a package / package (direct object)

The basic dictionary form is pako. It becomes pakon because it is what was received.

Ricevis is the past tense form of ricevi, meaning to receive.

Esperanto verb endings are very regular:

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

So:

• mi ricevas = I receive / am receiving
• mi ricevis = I received
• mi ricevos = I will receive

That makes Mi ricevis pakon straightforwardly I received a package.

Literally, per la poŝto means by means of the mail / through the postal service.

The preposition per often means by, by means of, or via. It shows the method or means used to do something.

Examples:

• Mi sendis ĝin per retpoŝto. = I sent it by email.
• Li venis per aŭto. = He came by car.
• Mi ricevis pakon per la poŝto. = I received a package by mail.

So English in the mail is not translated word-for-word here. Esperanto uses per to express the means.

In per la poŝto, la poŝto refers to the postal system or the mail as a service, not to one specific letterbox or one individual item of mail.

So la is natural here, much like talking about an institution or established system.

Compare:

• poŝto = mail, post, postal service
• la poŝto = the mail / the postal service

This is similar to how Esperanto often uses la for something understood as a known system or category.

It can refer to the general idea of mail/post or the postal service, depending on context.

In this sentence, per la poŝto means by mail or through the postal service.

But in other contexts:

• Mi iris al la poŝto. could mean I went to the post office.
• La poŝto malfruis. = The mail was delayed.

So the exact English translation depends on context.

Yes, poŝte is possible in many contexts and means by post / by mail.

For example:

• Mi ricevis pakon poŝte.

That is understandable and natural.
However, per la poŝto is also very common and very clear.

The difference is mainly stylistic:

• poŝte = more compact
• per la poŝto = a little more explicit

Because the speaker is introducing a package, not the package.

In Esperanto, la is the definite article, like English the.
There is no separate word for a/an. If there is no article, the noun is often indefinite.

So:

• Mi ricevis pakon. = I received a package.
• Mi ricevis la pakon. = I received the package.

The sentence uses pakon, not la pakon, because it is talking about a package without identifying it as a specific already-known one.

It tells when the action happened: this morning / this morning today.

It is made of:

• hodiaŭ = today
• matene = in the morning / morning-time

Together:

• hodiaŭ matene = this morning / today in the morning

This is a very normal Esperanto time expression.

Because matene is an adverb, while mateno is a noun.

• mateno = morning
• matene = in the morning / morningly, at morning time

The ending -e makes it adverbial.

So:

• hodiaŭ mateno by itself would not fit naturally here.
• hodiaŭ matene works because it describes when the receiving happened.

This is very common in Esperanto:

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

Yes. Esperanto word order is fairly flexible, as long as the sentence remains clear.

All of these are possible:

• Mi ricevis pakon per la poŝto hodiaŭ matene.
• Hodiaŭ matene mi ricevis pakon per la poŝto.
• Mi hodiaŭ matene ricevis pakon per la poŝto.

They all mean essentially the same thing, but the emphasis may shift slightly.

For example:

• Hodiaŭ matene... puts the time first, so it feels more prominent.
• Putting it at the end is also perfectly natural.

It is natural Esperanto.

A native English speaker might feel tempted to translate I got a package in the mail this morning word-for-word, but Esperanto often chooses structures that are natural to Esperanto rather than copying English exactly.

So this sentence is not awkwardly literal. It is a normal, idiomatic way to say it.

The letter ŝ is pronounced like English sh.

So:

• poŝto sounds roughly like POSH-to

More exactly:

• po as in po from pole without the glide
• ŝ = sh
• to with a clear t

Esperanto spelling is very regular, so once you know ŝ = sh, the word is easy to read.

Yes. That is just a difference in English wording.

Depending on the variety of English, you might translate:

• pako as package or parcel
• poŝto as mail or post

So the sentence could be rendered as:

• I received a package in the mail this morning.
• I got a parcel by post this morning.

The Esperanto sentence itself stays the same.