Obnova kupaonice traje dulje nego što smo mislili.

Because obnova usually takes the genitive case: obnova čega? = renovation of what?

So:

• kupaonica = bathroom
• kupaonice = of the bathroom (genitive singular)

That is why Obnova kupaonice means the renovation of the bathroom or more naturally the bathroom renovation.

Obnova means renewal, restoration, or renovation, depending on context.

In this sentence, obnova kupaonice is a natural way to say bathroom renovation.

Related words:

• obnova = renovation / restoration / renewal
• renovacija = renovation
• adaptacija = remodeling / adaptation
• remont = overhaul, repair work, often more technical

So obnova is a normal and idiomatic choice here.

Yes, English often uses the progressive: is taking longer.

Croatian usually does not need a separate progressive form. The simple present can cover an ongoing action or situation.

So:

• traje = lasts / is lasting / is taking
• Obnova kupaonice traje... = The bathroom renovation is taking...

This is very normal in Croatian. You do not need a form equivalent to English is taking.

Here dulje is an adverb, and it means longer.

It comes from dugo = for a long time / long (adverb).

Comparison:

• dugo = long / for a long time
• dulje = longer

Because it modifies the verb traje (lasts / takes), an adverb is needed:

• traje dugo = lasts a long time
• traje dulje = lasts longer

Yes. Dulje and duže are both used and both mean longer.

So these are both possible:

• traje dulje nego što smo mislili
• traje duže nego što smo mislili

Learners will often see dulje presented as the standard comparative of dugo, but duže is also very common in everyday speech.

In Croatian, when a comparison is followed by a whole clause, you often get:

• nego što...

Here the full comparison is:

• dulje nego što smo mislili = longer than we thought

So:

• nego introduces the comparison
• što helps connect it to the following clause

This structure is very common after comparatives.

Compare:

• dulje nego jučer = longer than yesterday
  no full clause, so no što
• dulje nego što smo mislili = longer than we thought
  full clause, so nego što

Smo mislili is the past tense (the Croatian perfect).

It is built with:

• the auxiliary smo = we have
• the past participle mislili = thought

So literally it is something like:

• we have thought

But in normal English translation here, it becomes:

• we thought

Croatian past tense commonly works this way:

• sam mislio/mislila = I thought
• si mislio/mislila = you thought
• je mislio/mislila = he/she thought
• smo mislili/mislile = we thought
• ste mislili/mislile = you all thought
• su mislili/mislile = they thought

Because Croatian often leaves subject pronouns out when they are already clear from the verb form.

In smo mislili, the verb already tells you the subject is we.

So Croatian prefers:

• ...nego što smo mislili

rather than:

• ...nego što mi smo mislili

The pronoun mi would only be added for emphasis or contrast.

The past participle in Croatian agrees with the subject in gender and number.

For we:

• mislili = masculine plural or mixed group, and often the default form
• mislile = feminine plural

So if the speakers are all women, you could say:

• nego što smo mislile

If the group is mixed or male, or if the masculine form is being used as the default, then:

• nego što smo mislili

That is why mislili appears here.

The given word order is the most natural and neutral:

• Obnova kupaonice traje dulje nego što smo mislili.

Croatian word order is fairly flexible, but changes usually add emphasis or sound less neutral.

For example, you might move things around in speech for emphasis, but the original version is the safest one for learners.

Also note that smo is a clitic, so it tends to come early in its clause:

• nego što smo mislili

That placement is normal and important.

You could form a sentence with trajanje, but it would usually sound more abstract or formal.

Compare:

• Obnova kupaonice traje dulje... = The bathroom renovation is taking longer...
• Trajanje obnove kupaonice... = The duration of the bathroom renovation...

So obnova ... traje is much more natural in everyday language.

Here are the main dictionary forms:

• obnova → noun, renovation / restoration
• kupaonica → noun, bathroom
• trajati → verb, to last / to take
• dugo → adverb, long / for a long time
• misliti → verb, to think

And their forms in the sentence are:

• obnova = nominative singular
• kupaonice = genitive singular
• traje = 3rd person singular present of trajati
• dulje = comparative adverb of dugo
• smo mislili = 1st person plural past tense of misliti

A natural way would be:

• Obnova naše kupaonice traje dulje nego što smo mislili.

Here naše means our, and it agrees with kupaonice in case, number, and gender.

So:

• naša kupaonica = our bathroom
• obnova naše kupaonice = the renovation of our bathroom

That is the most direct way to add our.