Mi skribas retpoŝton per komputilo.

Because retpoŝton is the direct object of skribas: it is the thing being written.

In Esperanto, direct objects take the accusative ending -n. The basic noun is retpoŝto, and as a direct object it becomes retpoŝton.

So:

• retpoŝto = email
• retpoŝton = email as the thing someone is writing, seeing, sending, etc.

Because komputilo is not the direct object. It comes after the preposition per, and after a preposition the noun normally does not take -n.

Here:

• retpoŝton = the thing being written
• per komputilo = by means of a computer / using a computer

So only retpoŝton gets the accusative ending.

The ending -as marks the present tense.

So skribas means write / am writing / do write, depending on context.

A useful point for English speakers: Esperanto does not usually make a separate grammatical distinction between I write and I am writing. The present tense -as can cover both.

Also, the verb ending stays the same for every person:

• mi skribas = I write / am writing
• vi skribas = you write / are writing
• li skribas = he writes / is writing

Per means by means of, using, or with the help of. It shows the tool or method used to do something.

So per komputilo means using a computer.

This is different from kun, which usually means with in the sense of together with or accompanied by.

Compare:

• Mi skribas per komputilo. = I write using a computer.
• Mi skribas kun amiko. = I write with a friend.

Often, yes. English uses on in many places where Esperanto chooses a more literal meaning.

If you mean using a computer as the tool, per komputilo is very natural.

Other prepositions would change the meaning:

• ĉe komputilo = at a computer
• sur komputilo = on top of a computer, or sometimes on the computer system/interface depending on context
• en komputilo = inside a computer

So in this sentence, per is the best choice because the computer is the means used to write the email.

Esperanto has no indefinite article. There is no separate word for a or an.

It does have a definite article: la, meaning the.

So:

• Mi skribas retpoŝton. = I am writing an email / I am writing email
• Mi skribas la retpoŝton. = I am writing the email

Without la, the noun is usually indefinite or simply not specified as definite.

The suffix -il- means tool, instrument, or device used for doing something.

So komputilo is built from the idea of computing plus -il-, giving the meaning a computing device.

This is a very common Esperanto pattern. For example:

• tranĉi = to cut → tranĉilo = a cutting tool, a knife
• skribi = to write → skribilo = a writing instrument

So komputilo is a nicely regular word.

Yes. Esperanto very often builds words by combining roots into a single word.

Retpoŝto is made from:

• ret- = net, network, internet-related
• poŝto = post, mail

So retpoŝto literally means something like internet mail, that is, email.

This kind of compounding is very normal in Esperanto.

Yes. Esperanto word order is more flexible than English word order because endings help show each word’s role.

The most neutral order here is:

Mi skribas retpoŝton per komputilo.

But you could also say things like:

• Retpoŝton mi skribas per komputilo.
• Per komputilo mi skribas retpoŝton.

These may sound more marked or emphasize a different part of the sentence, but they are still understandable because -n shows that retpoŝton is the object.

Usually, no. In normal Esperanto sentences, you usually keep the subject pronoun.

That is because skribas by itself does not tell you who is doing the action. The same form works for I, you, he, she, we, and they. So mi is important for clarity.

In special contexts, such as diaries, notes, or when the subject is already extremely obvious, people may omit it, but that is not the normal beginner pattern.

Ŝ is pronounced like English sh in ship.

So retpoŝton sounds roughly like reht-POSH-ton.

A few helpful pronunciation notes for the whole sentence:

• Mi ≈ mee
• skribas ≈ SKREE-bahs
• retpoŝton ≈ reht-POSH-ton
• per ≈ pehr
• komputilo ≈ kom-poo-TEE-lo

Also, Esperanto stress normally falls on the second-to-last syllable of each word, which is why:

• skribas → SKRI-bas
• retpoŝton → ret-POŜ-ton
• komputilo → kom-pu-TI-lo