Moram odmrznuti piletinu prije večere, jer je još u zamrzivaču.

Moram means I must / I have to. Croatian usually does not need the subject pronoun ja because the verb ending already shows the person.

• moram = I must
• moraš = you must
• mora = he/she/it must

You can say Ja moram for emphasis, but in a neutral sentence Moram is more natural.

Odmrznuti is the infinitive meaning to defrost / thaw out. It is a perfective verb, so it presents the action as something to be completed.

In this sentence, the idea is not just to be defrosting chicken, but to get the chicken defrosted before dinner. That is why odmrznuti fits well.

A related imperfective verb is odmrzavati, which focuses more on the process or repeated action.

Because piletinu is the accusative singular form of piletina.

Here, piletina is the direct object of odmrznuti:

• piletina = nominative
• piletinu = accusative

For many feminine nouns ending in -a, the accusative singular ends in -u.

Piletina usually means chicken meat or chicken as food, not a live chicken.

So in this sentence it most naturally means something like chicken meat that is frozen and needs to be defrosted before dinner.

If you wanted to talk about a live chicken, you would usually use a different word, such as kokoš or pile, depending on the meaning.

Because prije requires the genitive case.

So:

• večera = dinner, supper
• večere = genitive singular

That is why prije večere means before dinner.

This is a very common pattern in Croatian:

• prije ručka = before lunch
• prije škole = before school

Jer means because, and je means is.

So the second clause is basically:

• jer je još u zamrzivaču = because it is still in the freezer

The subject is not repeated, because it is understood from context. The sentence is talking about the chicken, so Croatian does not need to repeat piletina there.

Also, je is a clitic, and clitics usually appear near the beginning of the clause, which is why jer je sounds natural.

Here još means still.

So još u zamrzivaču means still in the freezer.

Be careful, because još can also mean other things depending on context, such as more or another:

• Još vode, molim. = More water, please.
• Još jedan. = One more.

But in your sentence, still is the correct meaning.

Because u with a location takes the locative case.

Here the chicken is located in the freezer, so:

• zamrzivač = freezer
• u zamrzivaču = in the freezer

If you were talking about motion into the freezer, Croatian would use the accusative instead:

• staviti u zamrzivač = to put into the freezer

So the contrast is:

• u zamrzivaču = in the freezer
• u zamrzivač = into the freezer

Croatian word order is fairly flexible. The sentence as given is neutral and natural:

• Moram odmrznuti piletinu prije večere, jer je još u zamrzivaču.

But other orders are also possible, depending on emphasis:

• Prije večere moram odmrznuti piletinu...
• Piletinu moram odmrznuti prije večere...

What is less flexible is the placement of short clitic forms like je, which usually need to stay near the beginning of their clause.

Because jer introduces a subordinate clause, and in Croatian that clause is normally separated by a comma.

So:

• Moram odmrznuti piletinu prije večere, jer je još u zamrzivaču.

This is standard punctuation. It works much like English punctuation before because in some sentence structures, although the rules are not identical in every case.