Ako nam ponestane vode, kupit ćemo još jednu litru u trgovini.

Because after ako when you talk about a future possibility, Croatian normally uses the present tense, not the future.

So:

• Ako nam ponestane vode = If we run out of water
• not usually Ako će nam ponestati vode

Also, ponestati is a perfective verb, so its present-tense form often refers to a future completed event in this kind of sentence.

Ponestati means to run short of or to run out of.

In this sentence, Ako nam ponestane vode means If we run out of water.

It is slightly different from nestati, which usually means to disappear.
So:

• nestati = disappear, vanish
• ponestati = run out, not have enough left

Nam is the dative form of mi (we/us).

With ponestati, Croatian uses the person affected in the dative:

• mi je ponestalo vremena = I ran out of time
• nam je ponestalo vode = we ran out of water

So nam here means something like to us or for us, though in natural English we just say we.

Because ponestati takes the thing that is lacking in the genitive.

So:

• voda = nominative, basic dictionary form
• vode = genitive singular

That is why Croatian says:

• ponestane vode = water runs out / we run out of water

This is very common with verbs and expressions involving absence, shortage, or quantity.

Ponestane is the 3rd person singular present form of ponestati.

Even though it is present in form, here it refers to the future because of the structure with ako.

So literally it is present, but functionally it means:

• if water runs out for us
• natural English: if we run out of water

In standard Croatian future tense, when the auxiliary ću, ćeš, će, ćemo, ćete, će comes after the infinitive, the final -i of infinitives ending in -ti is dropped in writing.

So:

• kupiti
  • ćemo
• becomes kupit ćemo

This is standard.

You can also place the auxiliary before the infinitive:

• Mi ćemo kupiti...

But kupiti ćemo is generally considered non-standard in formal Croatian.

It is the future tense: we will buy.

Breakdown:

• kupit = shortened infinitive stem used before the future auxiliary
• ćemo = we will

So:

• kupit ćemo = we will buy

Here još means more / another, not still.

So:

• još jednu litru = one more liter / another liter

Word by word:

• još = more, another
• jednu = one in feminine accusative singular
• litru = liter in feminine accusative singular

Since litra is feminine, both words have feminine forms.

Because they are the direct object of kupiti (to buy).

We are buying what?

• još jednu litru

So Croatian uses the accusative:

• jedna litra = nominative
• jednu litru = accusative

It is simply omitted because it is already understood from the first part of the sentence.

So:

• kupit ćemo još jednu litru
  naturally means
• we’ll buy another liter (of water)

If you want to be completely explicit, you could say:

• kupit ćemo još jednu litru vode

Both are understandable; the version without vode sounds natural when the context is clear.

Because u can take different cases depending on meaning:

• u + locative = location, in/at
• u + accusative = motion toward, into/to

Here the meaning is location:

• kupit ćemo ... u trgovini = we’ll buy ... at/in the store

Compare:

• Idemo u trgovinu. = We’re going to the store.
  motion, so accusative
• Kupujemo u trgovini. = We buy in/at the store.
  location, so locative

Because nam is a clitic, and Croatian clitics usually go near the beginning of the clause, often in the second position.

So:

• Ako nam ponestane vode sounds natural

Clitics such as mi, ti, se, ga, je, smo, ste, će, ćemo usually cannot stand just anywhere in the sentence.

This early placement is a normal Croatian word-order pattern.

Because the sentence begins with a subordinate if-clause:

• Ako nam ponestane vode, ...

Then comes the main clause:

• kupit ćemo još jednu litru u trgovini

When the dependent clause comes first, Croatian normally separates it with a comma, just like English often does:

• If we run out of water, we’ll buy another liter at the store.