Ne mogu odgonetnuti rebus, a kamoli dešifrirati šifru.

A kamoli is an idiomatic expression meaning let alone, much less, or never mind.

It usually comes after a negative statement and introduces something even less likely, harder, or more extreme.

So the logic here is:

• I can't do the first thing.
• Let alone the second, even harder thing.

A very common pattern is:

• negative statement + a kamoli + stronger/harder item

Examples:

• Ne razumijem članak, a kamoli knjigu.
  = I don't understand the article, let alone the book.

• Nemam vremena za ručak, a kamoli za odmor.
  = I don't have time for lunch, let alone for a break.

Croatian often leaves subject pronouns out because the verb usually tells you the person and number.

So:

• Ne mogu = I can't

Adding ja is possible, but it is usually only for emphasis, contrast, or clarity:

• Ja ne mogu odgonetnuti rebus...
  = I can't solve the rebus...

One small detail: mogu can also mean they can in isolation, so context matters. In a real conversation or with a translation provided, that is usually not a problem.

Because moći (to be able to / can) is a modal verb, and modal verbs in Croatian are normally followed by an infinitive.

So:

• mogu odgonetnuti
• mogu dešifrirati

This works much like English:

• can solve
• can decipher

You do not need an extra word like English to.

Because Croatian often leaves out repeated words when they are already understood from the first part of the sentence.

Here, the idea of can't is carried over automatically:

• Ne mogu odgonetnuti rebus, a kamoli dešifrirati šifru.

This is very similar to English:

• I can't solve a rebus, let alone decipher a cipher.

English also does not repeat can there.

So after a kamoli, Croatian often gives only the new element and leaves the rest understood.

Both words are direct objects, so both are in the accusative singular. The forms look different because the nouns belong to different declension patterns.

• rebus is a masculine inanimate noun

  • nominative: rebus
  • accusative: rebus

• šifra is a feminine noun ending in -a

  • nominative: šifra
  • accusative: šifru

So:

• odgonetnuti što? → rebus
• dešifrirati što? → šifru

A useful rule:

• many masculine inanimate nouns have the same nominative and accusative singular
• feminine nouns in -a usually change -a to -u in the accusative singular

They are related, but they are used for different kinds of things.

• odgonetnuti = to solve, figure out, unravel

  • used for puzzles, riddles, mysteries, hidden meanings
  • very natural with rebus

• dešifrirati = to decipher, decode

  • used for codes, ciphers, secret writing, encoded messages
  • very natural with šifra

So the pairings in the sentence are very natural:

• odgonetnuti rebus
• dešifrirati šifru

Because odgonetnuti focuses on the completed result: to solve it successfully.

That fits this sentence well. The speaker is not talking about the process of trying, but about not being able to reach the result.

Compare:

• Ne mogu odgonetnuti rebus.
  = I can't solve the rebus.

• Ne mogu odgonetavati rebus.
  = I can't be trying to solve the rebus / I can't spend time working it out.

The second version is much less natural here.

So for this sentence, odgonetnuti is the better choice.

Yes. In this sentence, dešifrirati is understood in a result-oriented way: to decipher successfully.

That matches the structure of the whole sentence:

• first, the speaker cannot achieve one result
• then, with a kamoli, they say they certainly cannot achieve an even harder result

So even if you do not want to get deeply into aspect yet, the main point is simple: here the verb means manage to decipher, not just be in the process of deciphering.

No. It can be followed by different kinds of words or phrases, depending on what is being contrasted.

For example:

• Ne razumijem ni ovo, a kamoli ono.
  = I don't understand even this, let alone that.

• Nemam za kavu, a kamoli za večeru.
  = I can't afford coffee, let alone dinner.

• Ne mogu trčati, a kamoli igrati nogomet.
  = I can't run, let alone play football.

In your sentence, it is followed by an infinitive because the first part also has a verb phrase with an infinitive.

Yes, absolutely.

• riješiti rebus = solve a rebus
• odgonetnuti rebus = figure out / solve a rebus

Both are correct. The difference is mostly one of nuance:

• riješiti is a more general verb meaning solve
• odgonetnuti has a slightly more specific feel of figuring out something puzzling or hidden

So odgonetnuti rebus sounds especially fitting for a rebus or riddle.

Yes. The comma is normal and expected here:

• Ne mogu odgonetnuti rebus, a kamoli dešifrirati šifru.

It marks the shift from the main statement to the emphatic let alone part.

In writing, you will very often see a kamoli introduced by a comma after the first clause.