Nismo našli ključanicu, a kamoli odgonetnuli zrcalno pismo.

It is a fixed expression meaning let alone or much less.

It is used when the first thing did not happen, and the second thing would be even less likely, harder, or more extreme. In this sentence, the idea is:

• we did not even find the keyhole
• so of course we did not decipher the mirror writing

So a kamoli sets up a kind of scale: if the easier or more basic thing failed, the harder one definitely did not happen either.

Because Croatian often leaves out repeated words when they are already clear from the previous clause. After a kamoli, this kind of omission is very common.

So odgonetnuli here stands for the fuller idea da smo odgonetnuli or simply the repeated perfect construction understood from the first part.

A fuller version could be:

Mi nismo našli ključanicu, a kamoli da smo odgonetnuli zrcalno pismo.

The shorter version is more natural and compact.

They are past participles agreeing with an omitted subject mi = we.

In Croatian, past participles show:

• number
• and, in many forms, gender

So:

• našli / odgonetnuli = masculine plural or a mixed group
• našle / odgonetnule = all-female group

Because Croatian often drops subject pronouns, these endings help identify the speaker group.

Because ključanicu is the accusative singular, and it is the direct object of našli.

The basic dictionary form is ključanica. Its relevant forms are:

• nominative: ključanica
• accusative: ključanicu

Since the sentence says we didn’t find the keyhole, the noun has to be in the accusative.

Ključanica means keyhole.

It does not mean:

• ključ = key
• brava = lock

So the sentence is specifically talking about the hole or opening where the key would go.

Because pismo is a neuter singular noun, and Croatian adjectives must agree with the noun in gender, number, and case.

So:

• zrcalno pismo = neuter singular

Also, for many neuter singular nouns, nominative and accusative look the same, so zrcalno pismo does not change form here even though it is the object of the verb.

Compare:

• nominative: zrcalno pismo
• accusative: zrcalno pismo

Not in this sentence.

Pismo can mean different things depending on context, such as:

• letter
• writing
• script
• alphabet

In zrcalno pismo, it means something like mirror writing or text written in mirror form, not a letter sent by mail.

Odgonetnuti means to figure out, solve, decipher, or unravel.

It suggests that something is not immediately understandable and requires effort to interpret. That makes it a good choice with zrcalno pismo, because mirror writing is something you have to work out, not just read normally.

So it is a bit stronger and more puzzle-like than a simple verb such as pročitati = to read.

Because the sentence is about completed results, or rather the failure to achieve those results.

Perfective verbs fit meanings like:

• we did not manage to find it
• we did not succeed in deciphering it

Compare:

• Nismo našli = we didn’t find it / failed to find it
• Nismo nalazili = we weren’t finding it / we kept not finding it

The imperfective versions would suggest an ongoing or repeated process, which is not the main point here.

Not exactly.

In this sentence, a is best understood as part of the fixed expression a kamoli. You usually should not translate it separately word for word.

On its own, a can mean something like:

• and
• but
• while

depending on context. But here the whole chunk a kamoli works together as let alone.

Yes, both are possible.

For example:

Mi nismo našli ključanicu, a kamoli da smo odgonetnuli zrcalno pismo.

Adding mi makes we explicit, usually for emphasis. Adding da makes the second part feel more like a full clause.

The meaning stays basically the same. The original version is just shorter and more idiomatic.

Because it separates the first clause from the following clause or clause-like phrase.

In standard writing, a kamoli is usually set off after a comma when it introduces this kind of follow-up idea. The comma helps show the structure clearly:

• first statement
• then a stronger, less likely one introduced by a kamoli

So the punctuation here is normal.

Because naći is a verb with stem changes in different forms.

Some common forms are:

• infinitive: naći
• present: nađem, nađeš, nađe...
• past participle: našao, našla, našli

So našli is the normal plural past participle form. This is one of those verbs whose forms you mostly have to learn as part of its pattern, rather than building every form from the infinitive in a fully predictable way.