La firmao postulis atestilon.

The -n is the accusative ending, and here it marks the direct object.

In this sentence:

• la firmao = the company, the thing doing the action
• atestilon = the certificate, the thing being required

So -n tells you that the certificate is what the company required.

-is is the regular past-tense ending in Esperanto.

So:

• postuli = to require / to demand
• postulis = required / demanded

Esperanto verb endings are very regular, so -is always signals the past.

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

It does not have a separate indefinite article for a/an. So a bare noun can mean a or just the noun in a general sense, depending on context.

That means:

• la firmao = the company
• atestilon = a certificate

Firmao is a noun, and the ending -o shows that.

In Esperanto:

• -o = noun
• -a = adjective
• -e = adverb
• -i = infinitive verb

So as soon as you see firmao, you can tell it is a noun: company or firm.

It can be broken down like this:

• atest- = the root related to attesting or certifying
• -il- = instrument or means
• -o = noun
• -n = direct object marker

So atestilo is literally something like a means of attesting, which is why it means certificate or attestation document.

No. Postuli is stronger.

• peti = to ask for, request
• postuli = to demand, require, insist on

So this sentence suggests the company was not just politely asking. It was making the certificate a requirement.

Yes. Esperanto word order is fairly flexible, because endings help show each word’s job.

The most neutral order is:

• La firmao postulis atestilon.

But you could also say:

• Atestilon la firmao postulis.

That would put more emphasis on atestilon. The -n still shows it is the object.

Because singular nouns end in -o.

Compare:

• firmao = company
• firmaoj = companies
• atestilo = certificate
• atestiloj = certificates

In the sentence, atestilon is singular atestilo plus the accusative -n. So it means one certificate, not certificates.