Ako itko zna gdje su grablje, to je moj djed.

This is a very common Croatian pattern, just like in English: Ako itko zna..., to je... = If anyone knows..., it’s...

It is a rhetorical conditional. The speaker is not really saying maybe yes, maybe no. They are strongly pointing to one person as the best candidate. So the idea is:

• If anyone knows, my grandfather does.
• In natural English: If anyone knows where the rake is, it’s my grandfather.

Itko means anyone / anybody.

Croatian usually uses itko in:

• questions
• negatives
• conditional clauses
• other non-specific contexts

Because this sentence begins with Ako (if), itko is the natural choice.

Compare:

• Ako itko zna... = If anyone knows...
• Netko zna... = Someone knows...

So netko would sound more like you already assume there is some person, while itko fits the open-ended if anyone idea.

Because itko is grammatically singular.

Even though itko means anyone, Croatian treats it like a singular subject, so the verb is in the 3rd person singular:

• itko zna
• netko zna
• nitko ne zna
• svatko zna

This is similar to English If anyone knows..., not If anyone know...

Because gdje asks about location, while kamo asks about direction.

Here the idea is where the rake is / where the rakes are, so it is about location:

• Gdje su grablje? = Where is the rake? / Where are the rakes?

If you were asking where to something is being moved, then kamo would be more natural.

Because grablje is a plural-only noun in standard Croatian.

That means it behaves grammatically like a plural noun, even when it refers to a single object. So Croatian says:

• Grablje su ovdje.
• literally: The rake are here
• natural English: The rake is here

This is similar to English scissors, which also takes a plural verb:

• The scissors are on the table.

So in gdje su grablje, the verb su agrees with the grammatically plural noun grablje.

It can depend on context.

Grammatically, grablje is plural. But in meaning, it can refer to:

• a rake
• the rake
• rakes / the rakes

So Gdje su grablje? can mean:

• Where is the rake?
• or Where are the rakes?

Context tells you which meaning is intended.

Here to is a neuter demonstrative pronoun meaning something like that or, more naturally in English, it in it’s my grandfather.

After a clause like Ako itko zna..., Croatian often uses:

• to je + noun

So:

• Ako itko zna gdje su grablje, to je moj djed.
• literally: If anyone knows where the rake is, that is my grandfather.
• natural English: If anyone knows where the rake is, it’s my grandfather.

This to je structure is very natural for identifying someone.

Yes. You could say:

• Ako itko zna gdje su grablje, onda je to moj djed.

This means the same thing, but onda adds a slightly clearer then feeling:

• If anyone knows where the rake is, then it’s my grandfather.

In everyday Croatian, onda is optional here. The original sentence without it is completely natural.

Because after to je in this kind of identifying sentence, Croatian normally uses the nominative:

• To je moj djed.
• Ovo je moja sestra.
• To je moj prijatelj.

So moj djed is in the nominative because it is the noun phrase identifying who to refers to.

Croatian does not have articles like a and the.

So nouns appear without articles, and definiteness is understood from:

• context
• word order
• the situation
• sometimes demonstratives like taj, taj moj, te, etc.

That is why grablje can mean the rake, a rake, or the rakes, depending on context.

Yes, but the original version is very natural and idiomatic.

For example, you could also hear:

• Ako itko zna gdje su grablje, onda je to moj djed.
• Moj djed zna gdje su grablje.
  This is simpler, but it loses the If anyone knows... emphasis.
• Ako itko zna gdje su grablje, moj djed to zna.
  This is also possible, but it is a slightly different structure.

So the original sentence is a nice, natural way to emphasize my grandfather is the one who would know.

Because Ako itko zna gdje su grablje is a subordinate conditional clause, and to je moj djed is the main clause.

Croatian normally separates this kind of clause from the main clause with a comma:

• Ako dođeš, nazovi me.
• Ako itko zna gdje su grablje, to je moj djed.

So the comma is standard and expected here.