Imamo još jednu litru vode u hladnjaku.

Because jednu litru is in the accusative case (direct object) after imamo (we have).

• Nominative (dictionary/basic form): jedna litra
• Accusative (object): jednu litru
  Also, litra is feminine, so jedan/jedna/jedno becomes jednu in feminine accusative.

After a quantity/measure expression (like jednu litru, dvije litre, pola litre, etc.), Croatian usually puts the “substance” in the genitive:

• jedna litra vode = one liter of water
  So voda (nominative) changes to vode (genitive).
  If you said vodu, that would usually mean the water itself as a direct object (not “a liter of”): e.g. Pijem vodu (I drink water).

Još is flexible. In this sentence it means an additional amount: one more / another (as in “we still have an extra liter”).
It can also mean still in other contexts:

• Još sam ovdje. = I’m still here.
  Here, with a number/quantity (još jednu litru), it naturally reads as one more.

Because u + location takes the locative case: u hladnjaku = “in the fridge (location)”.
If you mean movement into the fridge, u takes the accusative:

• Stavljam vodu u hladnjak. = “I’m putting water into the fridge.”

Hladnjaku is locative singular of hladnjak (base form = nominative singular).
A quick mini-paradigm (singular):

• N: hladnjak
• G: hladnjaka
• D/L: hladnjaku
• A: hladnjak (inanimate, same as nominative)

Both are common:

• hladnjak is the standard Croatian word (often preferred in Croatia).
• frižider is also widely understood and used (more colloquial / common in some regions and across BCS varieties).
  Your sentence works with either: u hladnjaku / u frižideru (locative forms differ).

Croatian often uses imati (to have) where English might say “there is/are left”: Imamo još... is a very natural way to express that something remains available.
You can also use an “existential” structure:

• Ima još jedna litra vode u hladnjaku. = “There is still/there’s another liter of water in the fridge.”
  Imamo emphasizes “we (as a household/group) have it available.”

• još jednu litru = one more / an additional liter (adds to what you already have).
• drugu litru = the second/another (the other) liter, often implying a contrast with the first one or a specific “other” item in a set.
  In everyday “extra amount” contexts, još jednu is the default.

Yes—Croatian j is like English y in yes.

• jednu ≈ YED-noo (with a short e like in met)
  Croatian spelling is very phonetic: letters usually keep the same sound.