La min fidinta kolegino tuj rimarkis la eraron.

Fidinta is the active past participle of fidi (to trust).

It breaks down like this:

• fid- = the root trust
• -int- = active past participle
• -a = adjective ending

So fidinta means something like:

• having trusted
• who trusted
• in this sentence, more naturally: who had trusted

Because it describes kolegino, the whole phrase la min fidinta kolegino means the colleague who had trusted me.

Because min is the direct object of fidi.

In Esperanto:

• mi = I
• min = me

Since the colleague is the one doing the trusting, and me is the one being trusted, Esperanto uses the accusative:

• fidi iun = to trust someone
• therefore: fidinta min or min fidinta = having trusted me

So min is not connected to rimarkis; it belongs to the participial phrase built from fidi.

Because fidinta is being used like an adjective modifying kolegino.

Esperanto often places adjectives before the noun:

• bela domo = beautiful house
• laca viro = tired man
• fidinta kolegino = colleague who had trusted

And the whole phrase can include other words before that adjective:

• la min fidinta kolegino

Literally, this is something like:

• the me-trusting colleague
• more naturally: the colleague who had trusted me

This is a compact alternative to a relative clause like:

• La kolegino, kiu fidis min, tuj rimarkis la eraron.

That version is often easier for beginners.

They are accusative, but for different verbs.

1. min

This is the object of fidi:

• fidi iun = to trust someone

So in min fidinta, min means me, the person trusted.

2. eraron

This is the object of rimarkis:

• rimarki ion = to notice something

So:

• rimarkis la eraron = noticed the mistake

A useful way to see the structure is:

• La ... kolegino = the subject
• tuj rimarkis = immediately noticed
• la eraron = the object of noticed

Inside the subject phrase, there is also:

• min fidinta = who had trusted me

So the sentence contains two different object relationships.

Because Esperanto usually keeps the modifier close to the noun it describes.

Here, min fidinta is a participial adjective phrase describing kolegino, so putting it right before the noun is very natural:

• la min fidinta kolegino

This makes it clear that min fidinta belongs to kolegino.

You may sometimes see different word orders in Esperanto, because word order is fairly flexible, but this version is neat and standard.

For a learner, it may help to think of it as a single block:

• [la min fidinta kolegino] = the colleague who had trusted me

Yes.

That is an important reason for using -int-.

In Esperanto participles:

• -ant- = active, ongoing/present
• -int- = active, earlier/past
• -ont- = active, future

So fidinta shows that the trusting happened earlier than the main action:

• first: she trusted me
• then: she noticed the mistake

That is why English often translates this as:

• The colleague who had trusted me immediately noticed the mistake.

If it were fidanta, it would suggest the colleague who is/was trusting me.

Because kolegino is specifically feminine.

• kolego = colleague
• kolegino = female colleague

The suffix -in- marks female sex.

So the sentence is talking about a female colleague. If the person were male or unspecified in older, more traditional usage, you might see kolego instead.

Because it agrees with kolegino, and kolegino is the subject here.

Adjectives and participial adjectives agree with the noun they describe in:

• number
• accusative marking

Here:

• kolegino = singular, nominative
• so the modifier is also singular, nominative: fidinta

If the noun were accusative, then the participle would also take -n:

• Mi vidis la min fidintan koleginon.
• I saw the colleague who had trusted me.

There, both fidintan and koleginon are accusative.

Yes, absolutely.

A very natural alternative is:

• La kolegino, kiu fidis min, tuj rimarkis la eraron.

That means the same basic thing: The colleague who had trusted me immediately noticed the mistake.

Why use the participle version, then?

Because Esperanto often likes compact expressions, and participles are a common way to compress relative clauses.

So:

• la min fidinta kolegino
• = la kolegino, kiu fidis min

The participle version is shorter and more condensed, but the relative clause version is often easier for learners to understand at first.

Tuj means immediately, right away, or at once.

So:

• tuj rimarkis = immediately noticed

It tells you that the colleague noticed the mistake without delay.

It is grammatical and perfectly good Esperanto, but it is somewhat more compact and advanced than what beginners usually produce.

A native English speaker may find it unusual because English does not normally say:

• the me-having-trusted colleague

English strongly prefers a relative clause:

• the colleague who had trusted me

Esperanto allows this compressed participial style much more easily. So the sentence is normal, but it may feel slightly formal, literary, or simply more concise than beginner-level Esperanto.

For everyday clarity, many speakers would also be happy with:

• La kolegino, kiu fidis min, tuj rimarkis la eraron.