Mi trovis unu ekzempleron.

Because mi is the subject of the sentence: the person doing the action.

• mi = I
• min = me

In Mi trovis unu ekzempleron, I am the one who found something, so mi is correct.

Compare:

• Mi trovis lin. = I found him.
• Li trovis min. = He found me.

The -n ending marks the object, not the subject.

-is is the Esperanto ending for the past tense.

So:

• trovi = to find
• trovas = find / am finding
• trovis = found
• trovos = will find

A very useful point: Esperanto verbs do not change for person.

• mi trovis = I found
• vi trovis = you found
• li trovis = he found

The verb ending stays the same.

The -n marks the direct object: the thing that was found.

Here:

• Mi = subject
• trovis = verb
• unu ekzempleron = direct object

The base noun is ekzemplero.
When it is the direct object, it becomes ekzempleron.

This is one of the most important patterns in Esperanto.

Because in normal use, numerals like unu, du, tri usually stay unchanged before a noun. The noun shows the case ending instead.

So you get:

• unu ekzemplero
• unu ekzempleron
• du ekzempleroj
• du ekzemplerojn

For a beginner, the safe rule is:

• keep unu unchanged here
• put the -n on the noun: ekzempleron

Its basic meaning is one.

Esperanto has no indefinite article, so English a/an is often simply not translated at all.

That means:

• Mi trovis ekzempleron. = I found a copy.
• Mi trovis unu ekzempleron. = I found one copy.

So unu is used when the idea of one matters.

No. They are different words.

• ekzemplo = example
• ekzemplero = copy, specimen, individual item

So ekzemplero usually means a physical copy or single instance of something, such as a copy of a book, magazine, or document.

That is a very common learner mistake, because the words look similar.

Yes, to some extent.

The normal, neutral order is:

• Mi trovis unu ekzempleron.

But because -n marks the object, Esperanto allows some flexibility. For example:

• Unu ekzempleron mi trovis.

That gives more emphasis to unu ekzempleron.

Still, for ordinary statements, subject–verb–object is the safest and most natural order for learners.

In Esperanto, the stress is always on the second-to-last syllable.

So here you get:

• MI — only one syllable
• TRO-vis
• U-nu
• ek-zem-PLE-ron

That fixed stress pattern is very regular, which makes pronunciation easier than in English.