U hladnjaku je ostala još jedna litra vode.

Because u can take two different cases:

• locative for location: in / inside
• accusative for motion toward somewhere: into

Here the sentence describes where the water remained, not movement, so u hladnjaku is locative singular: in the fridge.

Compare:

• Voda je u hladnjaku. = The water is in the fridge.
• Stavio sam vodu u hladnjak. = I put the water into the fridge.

In Croatian past tense, the l-participle agrees with the subject in gender and number.

The subject here is još jedna litra vode. The head noun is litra, and litra is feminine singular, so the verb must also be feminine singular:

• ostala = feminine singular

Even though vode appears in the phrase, the subject is not voda by itself; it is one liter of water.

Compare:

• Ostala je jedna litra vode. → feminine, because litra is feminine
• Ostao je jedan sok. → masculine, because sok is masculine
• Ostalo je jedno pivo. → neuter, because pivo is neuter

After nouns of quantity or measure, Croatian usually puts the thing being measured in the genitive.

So:

• litra vode = a liter of water
• čaša vina = a glass of wine
• kilo kruha = a kilo of bread

Here voda becomes vode, which is the genitive singular form.

This is very common with mass nouns and measure expressions.

In this sentence, još jedna litra vode means something like:

• still one liter of water
• one more liter of water remained
• there was still one liter of water left

The word još often means still, yet, or more/another, depending on context.

So još jedna can mean:

• one more / another one
• or still one

In this sentence, because of ostala and the overall meaning, it is understood as still one liter was left.

Because je is a clitic, and Croatian clitics usually go in second position in the sentence.

The first whole unit here is U hladnjaku, so the clitic je comes right after it:

• U hladnjaku je ostala još jedna litra vode.

This does not mean it must come after the very first word. It comes after the first phrase/unit.

This is very normal Croatian word order.

Yes. Croatian word order is fairly flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• U hladnjaku je ostala još jedna litra vode.
  Emphasis on the location: in the fridge
• Još jedna litra vode je ostala u hladnjaku.
  Emphasis more on one liter of water
• Još je jedna litra vode ostala u hladnjaku.
  Slightly different rhythm/emphasis

The version you were given sounds natural and puts the fridge/location first.

Croatian does not need a dummy subject like English there.

English often says:

• There was one liter of water left in the fridge.

Croatian simply uses the real subject directly:

• U hladnjaku je ostala još jedna litra vode.

So there is no separate word corresponding to English existential there.

Here ostala comes from ostati, which often means:

• to remain
• to be left
• to stay behind

In this sentence, the best sense is to be left / to remain.

So it is not just physical staying in a place. It means that after some was used or after some time passed, one liter was still left.

Because ostati is the perfective verb, and it fits a single completed situation/result:

• ostati = to remain / to end up being left
• ostajati = to be remaining / to remain repeatedly or over a period

Here the sentence refers to a result: at that point, one liter was left. So ostala je is the natural choice.

If you used ostajala je, it would suggest an ongoing or repeated process, which does not fit as well here.

Yes, you could hear versions without jedna, especially in speech:

• Ostala je još litra vode.

That would still mean there’s still a liter of water left.

Adding jedna makes the amount more explicit and countable:

• još jedna litra vode = still one liter / one more liter

So jedna is natural here, but leaving it out is also possible in some contexts.

As a whole, jedna litra vode is in the nominative singular, because it is the subject of the sentence.

Inside that phrase:

• jedna agrees with litra → nominative feminine singular
• litra is nominative singular
• vode is genitive singular because it depends on the measure noun litra

So the structure is:

• [jedna litra] = subject
• [vode] = dependent genitive, of water