Kad bi baterija bila prazna, koristili bismo punionicu za mobitel.

Yes, kad usually means when, but kad bi is a common pattern for a hypothetical situation.

Here, Kad bi baterija bila prazna... means something like:

• If the battery were empty...
• If the battery became empty...
• sometimes even Whenever the battery was empty..., depending on context

So in this sentence, kad bi works very much like if in English, not like a simple time word when.

Because both clauses are in the conditional.

• bi bila = would be
• koristili bismo = we would use

The form changes according to the subject:

• bi = 3rd person singular
• bismo = 1st person plural

So:

• baterija bi bila = the battery would be
• mi bismo koristili / koristili bismo = we would use

This is very normal in Croatian conditional sentences.

Because baterija is a feminine singular noun.

Both words have to agree with baterija:

• bila = feminine singular form of the past participle of biti (to be)
• prazna = feminine singular adjective meaning empty

So:

• masculine: bio prazan
• feminine: bila prazna
• neuter: bilo prazno

Since baterija is feminine, the sentence needs bila prazna.

In Croatian, the conditional auxiliary often comes after the main participle:

• koristili bismo
• radili bismo
• išli bismo

This is a very standard word order.

You may also see other arrangements in real speech or with extra words added, for example:

• Mi bismo koristili...

But koristili bismo is completely normal and very common.

Because it is the direct object of koristiti (to use), so it has to be in the accusative case.

The base form is:

• punionica = nominative singular

The accusative singular is:

• punionicu

So:

• punionica = a charging station
• koristiti punionicu = to use a charging station

This is the usual feminine noun pattern:

• žena → ženu
• knjiga → knjigu
• punionica → punionicu

Because za here means for.

So punionica za mobitel means:

• a charging station for a mobile phone

Also, mobitel stays the same in form here because it is a masculine inanimate noun, and for many such nouns:

• nominative singular = mobitel
• accusative singular = mobitel

So even though za takes the accusative, the form looks unchanged.

It is understandable, but it may not be the most natural everyday choice.

A native speaker would very often say:

• punjač za mobitel = phone charger

Punionica more often suggests a charging station, especially a larger place or unit, often for electric vehicles.

So if you mean a normal charger cable/device for a phone, punjač za mobitel is usually more natural.

Yes, you often could.

• Ako bi baterija bila prazna, koristili bismo...
• Kad bi baterija bila prazna, koristili bismo...

Both can introduce a condition.

Very roughly:

• ako = more directly if
• kad bi = often has a slightly more hypothetical or situational feel, like if/when that happened

In many contexts, both are possible.

Because the sentence begins with a subordinate conditional clause:

• Kad bi baterija bila prazna, = dependent clause
• koristili bismo punionicu za mobitel. = main clause

In Croatian, when this kind of clause comes first, it is normally separated by a comma.

Yes, but that is normal.

Croatian often leaves out subject pronouns because the verb form already shows the person and number.

Here, bismo already tells you the subject is we.

So Croatian does not need mi unless you want emphasis:

• Koristili bismo punionicu... = We would use the charging station...
• Mi bismo koristili punionicu... = We would use the charging station... with extra emphasis on we

Koristiti means to use and is an imperfective verb. It fits well if you are talking about using something in a general or practical sense.

A perfective verb like upotrijebiti would sound more like:

• to use it once
• to make use of it as a completed action

So:

• koristili bismo = we would use
• upotrijebili bismo = we would use / employ, often with a stronger sense of a single completed act

In many situations, koristili bismo sounds more neutral and natural.