Što se posteljine tiče, bijela jastučnica mi je bolja od plave.

It means as for bedding, when it comes to bedding, or as far as bedding is concerned.

This is a very common Croatian expression used to introduce a topic. It is mostly something you learn as a fixed phrase:

što se + genitive + tiče

So the sentence is basically saying:

As for bedding, the white pillowcase is better for me than the blue one.

Because posteljine is genitive singular of posteljina.

The phrase ticati se or što se ... tiče requires the genitive case. So:

• posteljina = bedding, bed linen
• posteljine = of bedding / as for bedding

This is one of those structures where the case is controlled by the expression itself.

Se is a clitic that belongs to the verb/expression ticati se, which means to concern, to relate to, or to have to do with.

So in što se posteljine tiče, you should really think of se tiče as part of one set expression.

It is not translated word-for-word in natural English. You usually just render the whole phrase as:

• as for ...
• regarding ...
• when it comes to ...

Yes. Što se tiče posteljine is also very common and means the same thing.

So both of these work:

• Što se posteljine tiče
• Što se tiče posteljine

Both mean as for bedding / when it comes to bedding.

The version in your sentence places posteljine inside the expression, which sounds natural and idiomatic.

Because it is the subject of the sentence.

The core of the sentence is:

bijela jastučnica mi je bolja od plave

Here, bijela jastučnica is the thing being described as better, so it stays in the nominative:

• jastučnica = pillowcase
• bijela jastučnica = the white pillowcase

Also, bijela agrees with jastučnica because jastučnica is feminine singular.

Mi is the dative form of ja, and here it means something like:

• to me
• for me
• in my opinion

So:

bijela jastučnica mi je bolja od plave = the white pillowcase is better than the blue one for me = more naturally, I prefer the white pillowcase to the blue one

It does not mean possession here. It marks a personal point of view or preference.

Because Croatian has clitics, and they tend to appear in a fixed order near the beginning of a clause.

In this sentence:

bijela jastučnica mi je bolja od plave

the clitics are:

• mi = dative pronoun
• je = auxiliary / copular form

Croatian normally puts them together in a clitic cluster, and mi je is the natural order here.

So a learner should mostly get used to hearing and producing:

• mi je
• ti je
• mu je
• joj je

rather than trying to build the order from English logic.

That is because bolja is the comparative form of dobra.

Croatian uses an irregular comparative here:

• dobar = good
• bolji = better

Since jastučnica is feminine singular, the form is:

• dobra = good
• bolja = better

So:

• bijela jastučnica je dobra = the white pillowcase is good
• bijela jastučnica je bolja = the white pillowcase is better

Because od takes the genitive case.

After a comparative like bolja = better, Croatian often uses:

od + genitive

So:

• plava jastučnica = the blue pillowcase
• od plave jastučnice = than the blue pillowcase

In your sentence, the noun jastučnice is omitted because it is obvious from context, so only the adjective remains:

• od plave = than the blue one

Here it means the blue one.

Croatian often leaves out a repeated noun when it is clear from context. The full version would be:

bijela jastučnica mi je bolja od plave jastučnice

But since jastučnice would repeat information unnecessarily, Croatian simply says:

od plave

So plave is short for plave jastučnice.

Because Croatian case endings often overlap.

Both posteljine and plave happen to be in forms ending in -e, but they belong to different words and different patterns:

• posteljine = genitive singular of posteljina
• plave = genitive singular feminine of plava, standing for plave jastučnice

So the same ending does not automatically mean the exact same grammar function across all words. You always have to look at:

• the base word
• the gender
• the role in the sentence
• the word that governs the case

Usually, with a simple noun phrase like this, od + genitive is the most natural choice.

So:

bolja od plave = better than the blue one

Nego is more common when what follows is more clause-like, for example:

• bolja nego što sam očekivao = better than I expected

With just a noun phrase, od plave is the standard, straightforward form.

Yes. Croatian word order is fairly flexible, though some versions sound more natural than others depending on emphasis.

For example, you could also say:

• Što se tiče posteljine, bijela jastučnica mi je bolja od plave.
• Bijela jastučnica mi je bolja od plave, što se posteljine tiče.

The original sentence is natural because it first sets the topic posteljina, then gives the speaker's preference.

So the given order works well as:

1. topic: as for bedding
2. statement: I prefer the white pillowcase to the blue one