Ovaj problem je ozbiljniji nego što smo mislili.

Because problem is masculine singular nominative, and ovaj has to agree with it.

• ovaj = this for a masculine singular noun in the nominative
• problem is masculine in Croatian
• the whole subject is ovaj problem = this problem

If the noun changed, ovaj would change too:

• ovaj problem = this problem
• ova knjiga = this book
• ovo pitanje = this question

So the form ovaj is chosen because it matches problem.

In Croatian, problem is a masculine noun. Many nouns ending in a consonant are masculine, and problem follows that pattern.

That matters because words that describe or point to the noun must agree with it:

• ovaj problem
• ozbiljniji problem
• taj problem

So both ovaj and ozbiljniji are in masculine singular nominative because they refer to problem.

ozbiljniji is the comparative form of ozbiljan.

• ozbiljan = serious
• ozbiljniji = more serious

So:

• Ovaj problem je ozbiljan. = This problem is serious.
• Ovaj problem je ozbiljniji... = This problem is more serious...

Croatian often forms comparatives with a special adjective form rather than using a separate word like English more.

It ends in -i because it agrees with problem, which is masculine singular nominative.

In this sentence, ozbiljniji is a predicate adjective after je, but it still agrees with the noun:

• problem je ozbiljniji = the problem is more serious

If the noun were feminine or neuter, the ending would change:

• Situacija je ozbiljnija. = The situation is more serious.
• Pitanje je ozbiljnije. = The question is more serious.

je is the 3rd person singular present of biti = to be.

So:

• Ovaj problem je ozbiljniji = This problem is more serious

It links the subject ovaj problem with the description ozbiljniji.

This is the same basic pattern as in English:

• X is Y
• problem je ozbiljniji

This is a very common Croatian way to say than we thought after a comparative.

• nego = than
• što smo mislili = what / as we thought

Together, nego što smo mislili means:

• than we thought
• more literally: than what we thought

Croatian often uses nego što when the comparison is followed by a whole clause, not just a noun.

Compare:

• ozbiljniji nego što smo mislili = more serious than we thought
• ozbiljniji od ovog problema = more serious than this problem

So a useful rule is:

• od
  • noun phrase
• nego (što)
  • clause

Sometimes in informal speech you may hear shorter versions, but in standard Croatian nego što smo mislili is the clearest and most natural form here.

So for learners, it is safest to use:

• ozbiljniji nego što smo mislili

Think of it as a fixed comparison pattern when a full clause follows.

Here što does not feel like the full English question word what. Its job is to introduce the clause after nego.

So in this sentence, it is best understood as part of the comparison structure:

• nego što smo mislili = than we thought

A very literal translation might look like than what we thought, but the natural English translation is simply than we thought.

This is about Croatian clitic word order.

smo is a clitic, and clitics normally go in the second position in their clause.

In the clause što smo mislili:

• što comes first
• smo goes second
• mislili follows

So:

• što smo mislili = correct

Inside this clause, što mislili smo would be wrong.

In a different sentence, you can often say:

• Mislili smo... = We thought...

But after što, the placement što smo mislili is the natural one.

Because the subject is we, which is contained in smo.

The Croatian past tense uses:

• a form of biti: sam, si, je, smo, ste, su
• plus the past participle

Here:

• smo = we have / we did
• mislili = thought

Together:

• smo mislili = we thought

The form mislili is plural. With we, Croatian normally uses the plural participle.

Yes, but that is normal.

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

• smo mislili already tells you the subject is we
• so mi is unnecessary unless you want emphasis

Compare:

• što smo mislili = than we thought
• što smo mi mislili = than we thought, specifically we

The version without mi is the neutral, usual one.

The sentence has a very normal Croatian structure:

• Ovaj problem = subject
• je ozbiljniji = is more serious
• nego što smo mislili = than we thought

So the full sentence is:

• Ovaj problem je ozbiljniji nego što smo mislili.

Croatian word order is more flexible than English, but this version is neutral and natural.

Not perfectly.

A very literal translation would be something like:

• This problem is more serious than what we thought.

But the natural English translation is:

• This problem is more serious than we thought.

So the Croatian structure with nego što does not have to be translated piece by piece into equally visible English words. It is just the normal Croatian way to build this comparison.