Ta posteljina izgleda lijepo, ali ova plahta mi je mekša.

Posteljina is a general or collective word for bed linen / bedding. It can refer to the set of textiles used on a bed.

Plahta is one specific item from that set: a sheet.

So the sentence first talks about the bedding in general, then singles out one particular sheet.

Because posteljina is a collective noun in Croatian. English often uses a plural idea or a mass expression for this kind of meaning, but Croatian treats posteljina as a singular feminine noun.

That is why you get singular forms with it, such as ta posteljina and izgleda.

Croatian has a three-way demonstrative system:

• ovaj / ova / ovo = this
• taj / ta / to = that
• onaj / ona / ono = that over there

So:

• ta posteljina points to bedding already known, already mentioned, or somewhat less immediate
• ova plahta means this sheet, usually something closer to the speaker or the current focus

A native English speaker often notices that Croatian can be more precise about distance or discourse focus.

Croatian does not have articles like English a and the.

Instead, definiteness is usually understood from context. If the speaker wants to point something out more clearly, demonstratives like ta and ova can help do that job.

So in this sentence, ta and ova carry some of the pointing function that English might express with that and this.

Both are in the nominative singular feminine.

That is because both are the subjects of their clauses:

• Ta posteljina izgleda lijepo
• ova plahta mi je mekša

Also:

• mi is dative singular
• mekša agrees with plahta in gender and number

With verbs like izgledati (to look / seem), Croatian very often uses the neuter singular adjective form, which in many cases looks the same as an adverb.

So lijepo here is the normal pattern for saying that something looks nice.

You will see the same kind of thing in sentences like:

• Izgleda dobro.
• Zvuči čudno.
• Osjeća se loše.

For an English speaker, this can feel unusual, because English uses an adjective in looks nice, while Croatian often uses this neuter form.

Mekša is the feminine singular comparative form of mek / mekan (soft).

Because plahta is feminine singular, the comparative has to match it:

• meka plahta = a soft sheet
• mekša plahta = a softer sheet

Related forms:

• masculine: mekši
• feminine: mekša
• neuter: mekše

And the superlative would be najmekša = the softest.

Mi is the unstressed dative pronoun meaning to me.

Here it adds a personal point of view:

• ova plahta je mekša = this sheet is softer
• ova plahta mi je mekša = this sheet is softer to me / in my opinion / for my taste

So mi does not change the basic meaning much, but it makes the statement sound more personal and subjective.

Both mi and je are clitics: short unstressed words that have special placement rules.

In Croatian, clitics usually come near the beginning of the clause, after the first stressed element or phrase, and their internal order is fairly fixed.

So mi je is the normal order here:

• ova plahta mi je mekša

That sounds natural and standard.

Croatian comparatives do not always need the second item to be stated explicitly.

If the comparison is obvious from context, mekša by itself is enough.

Here the speaker probably means something like:

• softer than the other bedding
• softer than another sheet
• softer than expected

If you want to state the comparison explicitly, you can add od + genitive:

• Ova plahta mi je mekša od one.
  = This sheet is softer than that one.

Yes, Croatian word order is flexible, but not completely free.

This sentence is in a very natural neutral order:

Ta posteljina izgleda lijepo, ali ova plahta mi je mekša.

Other orders are possible if you want different emphasis, for example:

• Ta posteljina lijepo izgleda, ali ova plahta mi je mekša.
• Ali ova plahta mi je mekša.

What usually stays together is the clitic group mi je.

So the order can shift for style or emphasis, but the original version is a good standard model for learners.