Slon je bio veći nego što smo očekivali.

Yes. Je bio is the Croatian past tense, built from:

• je = present-tense auxiliary is
• bio = past participle been / was

So Slon je bio literally looks like The elephant is been, but in normal English it means The elephant was.

This is the standard way Croatian forms the past tense:

• on je bio = he was
• ona je bila = she was
• ono je bilo = it was

Because slon is masculine singular, the participle is bio.

Because the sentence needs the comparative form: bigger.

• velik / veliki = big
• veći = bigger

So:

• Slon je bio velik. = The elephant was big.
• Slon je bio veći... = The elephant was bigger...

Croatian comparatives are often irregular, and veći is the comparative of velik.

They are two forms of the same adjective.

• velik = the short form
• veliki = the long form

In many situations, both can mean big, but the short form is very common in predicate use, especially after to be:

• Slon je velik. = The elephant is big.

The comparative here is simply veći, regardless of whether the basic form is thought of as velik or veliki.

Nego što introduces the second part of a comparison, especially when a whole clause follows.

• veći nego što smo očekivali = bigger than we expected

Here, the part after than is not just a noun or pronoun. It is a full clause:

• smo očekivali = we expected

So nego što works like than what / than we had expected.

This is very natural in Croatian after comparatives:

• brže nego što sam mislio = faster than I thought
• teže nego što izgleda = harder than it looks

Sometimes yes, but nego što is very common and often sounds more complete when followed by a full clause.

Compare:

• veći nego brat = bigger than his brother
• veći nego što smo očekivali = bigger than we expected

With a noun phrase, nego alone is normal. With a clause, nego što is a very standard choice.

You may also see no što in more formal or literary language:

• veći no što smo očekivali

Because smo is a clitic, and Croatian clitics usually go in the second position of the clause.

In the clause:

• što smo očekivali

the first element is što, so the clitic smo comes right after it.

That is why što smo očekivali sounds natural.

If you said što očekivali smo, that would be wrong in standard Croatian.

Smo očekivali means we expected.

It is another past tense:

• smo = auxiliary for we
• očekivali = past participle of očekivati = to expect

So:

• mi smo očekivali = we expected

In the sentence, mi is omitted because Croatian often leaves subject pronouns out when they are already clear from the verb form.

Because the past participle agrees with the subject in gender and number.

• mi smo očekivali = we expected, if the group is masculine or mixed
• mi smo očekivale = we expected, if the group is all female

Textbooks often use the masculine/mixed plural as the default citation form, so očekivali is very common.

Slon is in the nominative singular because it is the subject of the sentence.

• slon = elephant
• nominative singular = the elephant, as the doer or topic of the verb

Since Croatian has no articles, slon can mean an elephant or the elephant, depending on context.

Because Croatian does not have articles like a/an/the.

So:

• slon can mean an elephant or the elephant

Context tells you which meaning is intended.

In this sentence, English uses the elephant, but Croatian simply says slon.

Here što does not mean a direct question word like what?. It is part of the fixed comparative structure nego što.

So in this sentence, it helps link the comparison:

• veći nego što smo očekivali = bigger than we expected

You can think of it as part of a connector rather than as a separate what.

The sentence is quite natural as written, but Croatian word order is more flexible than English.

The standard neutral order is:

• Slon je bio veći nego što smo očekivali.

You might move words for emphasis in other contexts, but the clitic placement still matters:

• Je and smo cannot just go anywhere

So while Croatian allows some flexibility, this version is the most neutral and useful one to learn first.

You can use that as a learning aid, but the best natural English translation is usually:

• The elephant was bigger than we expected.

A more literal breakdown is:

• Slon = elephant
• je bio = was
• veći = bigger
• nego što = than what / than
• smo očekivali = we expected

So the literal structure may help you understand the Croatian grammar, even if the final English translation is smoother without what.