Ne selimo se zato što nam je stari stan bio ružan, nego zato što nije imao dovoljno mjesta.

Because the verb here is seliti se / preseliti se, which means to move house / to relocate oneself.

• seliti without se usually means to move something or to relocate someone/something
• seliti se means to move oneself, so in English simply to move

So:

• Selimo namještaj. = We are moving the furniture.
• Selimo se. = We are moving.

The se is a reflexive particle and is a normal part of this verb.

Croatian often uses the pattern:

Ne X zato što..., nego zato što...

This means:

Not X because..., but because...

So the sentence does not mean they are not moving at all. It means:

• We are not moving because the old apartment was ugly
• but because it didn’t have enough space

The negation applies to the reason being rejected, not to the whole idea of moving.

They work together in a contrast structure.

• zato što = because
• nego = but rather / but instead

So:

Ne selimo se zato što..., nego zato što... = We’re not moving because..., but because...

This is a very common Croatian pattern when correcting an assumed reason and giving the real one.

Because nego is normally used after a negation when you are replacing one idea with another.

Here the structure is:

• not because A
• but because B

That kind of correction strongly prefers nego.

Compare:

• Nije crven, nego plav. = It’s not red, but blue.
• Ne radim danas, nego sutra. = I’m not working today, but tomorrow.

Ali is a more general but, but in this kind of not A, but B correction, nego is the natural choice.

Nam is the dative form of mi and here means to us / for us.

In the phrase:

nam je stari stan bio ružan

the most natural interpretation is something like:

• the old apartment was ugly to us
• in smoother English: we found the old apartment ugly or our old apartment was ugly

Croatian often uses the dative in ways that do not translate word-for-word into English.

Both are possible, but they are slightly different in emphasis.

• naš stari stan = our old apartment directly, with a possessive adjective
• stari stan nam je bio ružan uses nam and can sound a bit more personal, like the old apartment was ugly to us

In many situations Croatian prefers this dative style instead of using a possessive every time.

So:

• Naš stari stan je bio ružan. = Our old apartment was ugly.
• Stari stan nam je bio ružan. = The old apartment was ugly to us / Our old apartment seemed ugly to us.

Both are natural, but the sentence you were given is very idiomatic.

Bio is the past form of biti (to be), so it means was.

• je ružan = is ugly
• bio ružan = was ugly

The speaker is talking about the old apartment as it was before they moved or before the current situation, so the past is needed.

Because stan is masculine singular, the participle is bio.

Compare:

• stan je bio ružan = the apartment was ugly
• kuća je bila ružna = the house was ugly
• mjesto je bilo ružno = the place was ugly

Croatian normally uses a simple past form here, not a form like English had had or an extra form of to be.

• imao je = he/it had
• nije imao = he/it didn’t have

So:

nije imao dovoljno mjesta = it didn’t have enough space

This is the standard and natural way to say it.

Because dovoljno (enough) is followed by the genitive.

The noun mjesto becomes mjesta in the genitive singular here.

So:

• mjesto = space / place
• dovoljno mjesta = enough space

This is a very important Croatian pattern. Other quantity words often trigger the genitive too.

For example:

• puno vremena = a lot of time
• malo novca = little money
• dosta ljudi = enough / a lot of people

Here it means space, not place.

Literally, mjesto can mean place, spot, or space, depending on context. In housing contexts, imati dovoljno mjesta means to have enough room / enough space.

So in this sentence:

nije imao dovoljno mjesta means it didn’t have enough space

not it didn’t have enough places.

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

This version is natural because short unstressed words like nam and je tend to appear early in the sentence, in what is often called the second position area.

That is why you get:

što nam je stari stan bio ružan

rather than something less natural like:

što stari stan nam je bio ružan

Possible word orders can change emphasis, but the given one is very standard and idiomatic.

Because Croatian often uses the present tense for actions happening now or in the near future, just like English does in sentences such as We’re moving next week.

So:

• Selimo se. = We are moving / We’re moving house.

This can refer to a current process or a planned move. The rest of the sentence then explains the reason for that move.

In context, it usually means the old apartment in the sense of the apartment we lived in before.

Croatian stari can mean:

• physically old
• previous / former, depending on context

Here, because they are moving away from it, English often translates it best as:

• our old apartment
• the previous apartment

Even if the apartment was not literally very old, stari stan is still natural Croatian here.