Dok traje obnova, topla voda nije uvijek dostupna.

Here dok means while or for as long as. It introduces a time clause:

Dok traje obnova = While the renovation is going on

In other contexts, dok can also mean until, depending on the sentence, so learners often have to rely on context.

Traje is the 3rd person singular present tense of trajati, which means to last, to continue, or to be in progress.

So:

obnova traje = the renovation is lasting / continuing / is underway

In natural English, we usually translate this more smoothly as the renovation is ongoing or the renovation is in progress.

Both are possible.

Croatian word order is more flexible than English word order. The version in your sentence, Dok traje obnova, is a very natural way to say While the renovation is ongoing.

You could also say:

Dok obnova traje, topla voda nije uvijek dostupna.

That is grammatical too, but Dok traje obnova sounds a bit more natural and idiomatic in this kind of notice or announcement.

Because Dok traje obnova is a subordinate clause placed before the main clause.

So the sentence is structured like this:

Dok traje obnova, = subordinate time clause
topla voda nije uvijek dostupna. = main clause

This is similar to English punctuation in sentences like:

While the renovation is ongoing, hot water is not always available.

Obnova is in the nominative singular because it is the subject of traje.

The basic clause is:

obnova traje = the renovation lasts / is in progress

Since obnova is the thing doing the action of traje, it stays in the nominative.

Also, obnova is a feminine noun.

Topla voda means hot water.

The adjective topla means warm/hot, and voda means water. In Croatian, adjectives usually come before the noun, just like in English.

The adjective has to agree with the noun in gender, number, and case. Since voda is feminine singular nominative, the adjective is also feminine singular nominative:

topla voda

If the noun were masculine or neuter, the adjective form would change.

Because Croatian does not have articles like English a and the.

So:

obnova can mean a renovation or the renovation
topla voda can mean hot water or the hot water

The exact meaning comes from context. In a notice like this, English naturally uses the renovation and just hot water or the hot water, but Croatian does not need separate article words.

In Croatian, the negative forms of to be are written as single words:

nisam, nisi, nije, nismo, niste, nisu

So:

je = is
nije = is not

You do not normally write ne je.

Dostupna is the feminine singular form of the adjective dostupan, which means available or accessible.

It is feminine singular because it agrees with the subject topla voda, which is also feminine singular.

So:

topla voda ... nije dostupna = hot water ... is not available

This is a very common Croatian pattern: after biti (to be), the adjective still agrees with the subject.

Uvijek means always, and its position here gives the meaning not always available.

So:

nije uvijek dostupna = is not always available

This is important because it does not mean always unavailable. It means availability is inconsistent.

Word order can affect emphasis. For example:

Topla voda nije uvijek dostupna. = the normal, natural wording
A different placement could sound more marked or change the emphasis.

So the position of uvijek here is very natural and matches the English meaning closely.

No, not quite.

nije uvijek dostupna means it is not always available. Sometimes it is available, sometimes it is not.

uvijek nije dostupna would suggest it is always unavailable, which is much stronger and usually not what this sentence means.

So the original sentence is carefully worded to say that hot water may be unavailable at some times during the renovation, but not necessarily all the time.

Yes, absolutely.

That version is also correct and natural:

Topla voda nije uvijek dostupna dok traje obnova.

The meaning stays the same. The main difference is word order and focus:

Dok traje obnova, ... puts the time situation first.
Topla voda nije uvijek dostupna ... puts the main message first.

Both are common in Croatian.

It is an adjective.

In this sentence it works as a predicate adjective after nije:

topla voda nije dostupna = hot water is not available

So even though it comes after the verb, it is still an adjective agreeing with topla voda.

This is similar to English adjectives in sentences like:

The water is cold.
The service is available.

Yes. Croatian normally uses the present tense here.

Dok traje obnova literally uses the present tense traje, but it refers to a current ongoing situation: the renovation is happening now.

That is completely normal in Croatian. English often also uses the present in this kind of statement:

While the renovation is ongoing, hot water is not always available.

So there is nothing unusual about the tense choice here.