Poruka je napisana nevidljivom tintom.

Poruka is the subject of the sentence, so it is in the nominative singular.

• poruka = message
• It is a feminine noun

That feminine gender matters later, because the participle napisana has to agree with it.

Because napisana must agree with poruka.

In Croatian, this kind of participle changes for gender and number:

• napisan = masculine singular
• napisana = feminine singular
• napisano = neuter singular
• napisane / napisani / napisana = plural forms depending on gender

Since poruka is feminine singular, the correct form is napisana.

Je napisana is a passive/resultative structure built with:

• je = is / has been (3rd person singular of biti, to be)
• napisana = past passive participle / participial adjective from napisati

Together, je napisana means something like:

• is written
• has been written
• was written

Which English version fits best depends on context, but the Croatian structure itself is very natural here.

It comes from the verb napisati, which means to write down / to write completely.

This is the perfective verb. Croatian often uses the perfective form when the action is seen as completed.

Related pair:

• pisati = imperfective, to be writing / to write habitually
• napisati = perfective, to write, finish writing

So napisana suggests a completed result: the message ended up written.

Tintom is the instrumental singular of tinta.

• tinta = ink
• tintom = with ink

The instrumental case is often used to express the means or instrument by which something is done.

So:

• napisana tintom = written with ink

This is a very common use of the instrumental in Croatian.

Because Croatian usually uses the instrumental without a preposition when it means by means of / with the help of something.

So you say:

• pisati olovkom = to write with a pencil
• pisati kemijskom = to write with a pen
• pisati tintom = to write with ink

Using sa/s here would usually sound unnatural if you just mean the instrument used.

Because nevidljivom has to agree with tintom.

• tinta is feminine singular
• tintom is instrumental singular
• so the adjective must also be feminine singular instrumental

That gives:

• nevidljiva tinta = invisible ink (nominative)
• nevidljivom tintom = with invisible ink (instrumental)

So both words change together.

The dictionary form is nevidljiv = invisible.

Like many Croatian adjectives, it changes depending on the noun:

• nevidljiv = masculine singular nominative
• nevidljiva = feminine singular nominative
• nevidljivo = neuter singular nominative
• nevidljivom = instrumental singular here

Since it describes tintom, the form must match that noun.

Yes. Croatian word order is more flexible than English word order.

The neutral version is:

• Poruka je napisana nevidljivom tintom.

But you could also say:

• Nevidljivom tintom je napisana poruka.
• Poruka je nevidljivom tintom napisana.

These alternatives may sound more marked or emphasize different parts of the sentence, but they are still grammatical.

English is much more rigid, while Croatian uses word order more for focus and emphasis.

It can be possible, but it is not exactly the same in feel.

• napisana comes from the perfective napisati and emphasizes a completed result
• pisana comes from the imperfective pisati and is less focused on completion

So:

• Poruka je napisana nevidljivom tintom = the message was written / has been written in invisible ink, with a clear completed-result sense

Using pisana can sound more like describing the writing as a process, manner, or general characteristic, depending on context. In this sentence, napisana is the more natural choice.

Yes, it functions as a passive sentence.

The active version would be something like:

• Netko je napisao poruku nevidljivom tintom. = Someone wrote the message in invisible ink.

The passive version removes the doer and makes poruka the subject:

• Poruka je napisana nevidljivom tintom.

This is very common in Croatian, especially when the person who did the action is unknown or unimportant.

A simple pronunciation guide is:

POH-roo-kah yeh nah-PEE-sah-nah neh-vee-DLYEE-vohm TEEN-tohm

A few useful notes:

• j is pronounced like English y
• je sounds roughly like ye
• lj is a soft sound, somewhat like the lli in million for many learners
• stress can vary by dialect and teaching style, but this approximation is good enough for most beginners

This sentence is a great example of three key Croatian patterns:

1. Agreement

   • poruka is feminine singular
   • so napisana must also be feminine singular

2. Instrumental for means

   • tintom means with ink
   • no preposition is needed

3. Adjective agreement in cases

   • nevidljivom matches tintom in gender, number, and case

So if you understand why Croatian says:

• poruka → nominative subject
• napisana → agrees with the subject
• nevidljivom tintom → instrumental phrase meaning with invisible ink

then you have understood the main grammar of the whole sentence.