Ovo je zrcalno pismo.

Because pismo is a neuter singular noun, and Croatian demonstratives must agree with the noun they refer to.

• ovaj = masculine singular
• ova = feminine singular
• ovo = neuter singular

So:

• ovaj stol = this table
• ova knjiga = this book
• ovo pismo = this letter / this writing

In your sentence, ovo matches pismo.

It behaves like a demonstrative pronoun meaning this.

In Ovo je zrcalno pismo, ovo stands on its own as the subject: This is ...

Compare:

• Ovo je pismo. = This is a letter.
• Ovo pismo je dugo. = This letter is long.

In the second example, ovo directly modifies pismo, so it works more like this in this letter.

Je is the 3rd person singular present form of the verb biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In the sentence Ovo je zrcalno pismo, je means is.

In standard Croatian, no. You normally need je in a sentence like this.

• Ovo je zrcalno pismo. = correct full sentence
• Ovo zrcalno pismo. = not a full sentence; it just means this mirror writing as a noun phrase

So unlike in some languages where the present-tense to be can disappear, Croatian usually keeps it here.

Because zrcalno is an adjective agreeing with the neuter noun pismo.

Croatian adjectives change for gender:

• zrcalan = masculine
• zrcalna = feminine
• zrcalno = neuter

Since pismo is neuter, the adjective must also be neuter:

• zrcalno pismo

Pismo is neuter.

A very common clue is the ending -o or -e, which often marks neuter nouns in Croatian. So pismo fits that pattern.

Because it is neuter singular, other words around it must match:

• ovo pismo
• zrcalno pismo

They are in the nominative.

This is the basic dictionary form, used here because the sentence is identifying something:

• Ovo = nominative neuter singular
• zrcalno pismo = nominative neuter singular

So the structure is basically:

• This
  • is
    • mirror writing

That is a great question, because pismo has more than one meaning.

It can mean:

• letter (a written letter)
• script / writing system / writing style
• sometimes writing more generally, depending on context

In zrcalno pismo, it does not mean a letter you send in an envelope. Here it means something like mirror writing / mirrored script.

So context tells you which meaning is intended.

Yes, it works as a normal established phrase for mirror writing.

It is built from:

• zrcalo = mirror
• zrcalan / zrcalno = mirror-like, mirrored
• pismo = writing, script

So zrcalno pismo literally means something like mirrored writing.

Yes. They are directly related.

• zrcalo = mirror
• zrcalan = mirrored / mirror-like
• zrcalno = neuter form of that adjective

So the adjective comes from the noun zrcalo.

For many English speakers, the hardest part is the cluster zr at the start.

A rough guide:

• z like z in zoo
• r is a rolled or tapped Croatian r
• c in Croatian is pronounced ts
• je is roughly ye

So zrcalno is approximately ZR-tsal-no, and the whole sentence is roughly:

O-vo ye ZR-tsal-no PIS-mo

That is only an approximation, but it helps as a starting point.

The most neutral order is:

Ovo je zrcalno pismo.

Croatian word order is fairly flexible, but different orders create different emphasis.

For example, you might move things around in special contexts, but the sentence above is the normal, natural way to say it when simply identifying something.

Also note this important difference:

• Ovo je zrcalno pismo. = This is mirror writing.
• Ovo zrcalno pismo = this mirror writing

The second one is not a full sentence.

Because Croatian does not use articles the way English does.

English needs:

• This is a letter.
• This is the letter.

Croatian usually just says:

• Ovo je pismo.

Context tells you whether it is closer to a letter, the letter, or just letter/writing in a broader sense.

So the lack of a/the is completely normal.

Yes, often you can, but ovo and to are not exactly the same.

• ovo = this
• to = that / it / that thing

If something is being pointed out as this, ovo is the most direct choice.

So:

• Ovo je zrcalno pismo. = This is mirror writing.
• To je zrcalno pismo. = That is mirror writing. / That is mirror writing there. / It is mirror writing.

Which one sounds best depends on context.