Atentu la lastan detalon en la raporto; mi ne rimarkis ĝin hieraŭ.

The ending -u is the Esperanto verb form for a command, request, wish, or instruction.

So atentu means something like pay attention or be attentive.

In this sentence, the subject vi is understood, so Esperanto does not need to say it explicitly.

• Atentu... = Pay attention...
• Vi atentu... = also possible, but more emphatic

It can be either.

Esperanto uses vi for both singular you and plural you, and the -u form does not change for number. So atentu can mean:

• pay attention to one person
• pay attention to several people

Only context tells you which is meant.

Because la lastan detalon is the direct object of atentu.

In Esperanto, direct objects usually take -n. Also, adjectives must agree with the nouns they describe, so if the noun has -n, the adjective gets -n too.

Breakdown:

• detalo = detail
• detalon = detail as a direct object
• lasta = last, final
• lastan = adjective agreeing with detalon

So:

• la lasta detalo = the last detail
• la lastan detalon = the last detail as the object

Because en la raporto expresses location, not motion.

In Esperanto, after a preposition like en, you normally do not use -n if you are talking about where something is.

Here the meaning is in the report, so:

• en la raporto = in the report

If there were movement into something, then -n could appear:

• en la raporton = into the report

So this sentence uses en la raporto because the detail is located there.

Ĝin refers back to la lastan detalon.

The basic pronoun is ĝi, which is used for a thing, an animal, or something non-personal. Since it is the direct object of rimarkis, it takes -n:

• ĝi = it
• ĝin = it as a direct object

So mi ne rimarkis ĝin means I did not notice it, where it = the last detail.

Because -is is the Esperanto past-tense ending.

The verb is:

• rimarki = to notice

Its forms include:

• rimarkas = notice / am noticing
• rimarkis = noticed
• rimarkos = will notice

So mi ne rimarkis ĝin hieraŭ shows that the not-noticing happened in the past.

In Esperanto, ne usually goes directly before the word or phrase it negates. Most often, that is the verb.

So:

• mi ne rimarkis ĝin = I did not notice it

That is the normal, neutral placement.

If ne is placed elsewhere, it can change the emphasis, but ne before the verb is the standard pattern learners should use first.

They are related, but they are not the same.

• atenti = to pay attention, to direct your attention carefully
• rimarki = to notice, to become aware of something

In this sentence:

• Atentu la lastan detalon... tells someone to focus on that detail.
• mi ne rimarkis ĝin hieraŭ says the speaker failed to notice it yesterday.

So one verb is about deliberate attention, and the other is about noticing something.

Yes, Esperanto word order is fairly flexible.

The given order is natural and clear:

• mi ne rimarkis ĝin hieraŭ

But other orders are also possible, for example:

• Hieraŭ mi ne rimarkis ĝin
• Mi hieraŭ ne rimarkis ĝin

Because ĝin has the object ending -n, Esperanto can still show its role even if its position changes.

That said, the original version sounds neutral and natural.

The semicolon links two closely related independent clauses:

• Atentu la lastan detalon en la raporto
• mi ne rimarkis ĝin hieraŭ

The two parts are strongly connected in meaning, so a semicolon works well. It is stronger than a comma and weaker than a full stop.

You could also separate them with a period:

• Atentu la lastan detalon en la raporto. Mi ne rimarkis ĝin hieraŭ.

The semicolon is mainly a punctuation choice, not a special grammar rule.