Learn Hardcore Croatian: Njen glas je lijep kad pjeva u parku. - Her voice is beautiful when she sings in the park.

njezin (and its forms njezina, njezino) is slightly more formal and is often recommended in grammar books.
njen (and njena, njeno) is very common in everyday speech and widely accepted.

So njen glas and njezin glas are both correct and mean her voice.

Why is it glas and not something like glasa or glasu in this sentence?

Here glas is the subject of the sentence, so it is in the nominative case (the “dictionary” form).
The structure is:

Njen glas (subject, nominative)
je (verb “to be”)
lijep (predicate adjective).

Forms like glasa, glasu are other cases (genitive, dative/locative) and are not used for the subject in this type of sentence.

Why do we need je here? Could we say Njen glas lijep kad pjeva u parku?

je is the 3rd person singular of the verb biti (to be) and works as the copula is:

Njen glas je lijep = Her voice is beautiful.

In careful standard Croatian you normally include je in such sentences.
In informal speech, people sometimes drop it (e.g. Glas joj lijep), but Njen glas je lijep kad pjeva u parku is the natural, standard version.

Why is it lijep and not lijepa or lijepo?

The adjective must agree in gender, number, and case with the noun it describes.

glas is masculine singular nominative.
The matching adjective form is lijep (masculine singular nominative).

If the noun were feminine, you’d say:

Njena pjesma je lijepa – Her song is beautiful.

For a neuter noun:

Njeno dijete je lijepo – Her child is beautiful.

What is the role of kad pjeva u parku in this sentence?

kad pjeva u parku is a time clause, meaning when she sings in the park.
It tells you under what circumstances her voice is beautiful.
So the whole structure is:

Main clause: Njen glas je lijep (Her voice is beautiful)
Time clause: kad pjeva u parku (when she sings in the park).

Why is there no word for she in kad pjeva u parku?

Croatian is a “pro‑drop” language: subject pronouns (like ona = she) are usually omitted when the subject is clear from context.
From Njen glas we know we are talking about her, so the subject of pjeva is understood as she.
If you wanted to be very explicit, you could say kad ona pjeva u parku, but that usually sounds unnecessary or emphatic.

Why is it present tense pjeva when in English we say when she sings (which can be general, not just right now)?

Croatian uses the present tense much like English does for general truths and repeated actions.
kad pjeva u parku can mean:

whenever she sings in the park (habitual), or
at the time she is singing there (right now), depending on context.

So present pjeva covers both the “whenever she sings” and “when she is singing” ideas.

What is the difference between kad and kada?

kad and kada both mean when and are interchangeable in most contexts.

kada is a bit more formal or slightly more emphatic.
kad is shorter and very common in everyday speech.

You could say Njen glas je lijep kada pjeva u parku with no change in meaning.

Why is it u parku and not u park?

With the preposition u and a static location (in the park), Croatian uses the locative case.
The locative singular of park is parku:

nominative: park (subject)
locative: u parku (in the park).

u park (accusative) would usually mean into the park, expressing movement toward the park.

Could the word order change, for example Njen je glas lijep kad pjeva u parku or Lijep je njen glas kad pjeva u parku?

Yes, Croatian word order is fairly flexible, and both versions are possible:

Njen je glas lijep kad pjeva u parku – often used to slightly highlight glas.
Lijep je njen glas kad pjeva u parku – highlights lijep (the quality “beautiful”).

The basic, neutral order is Njen glas je lijep kad pjeva u parku, but reordering is common for emphasis or style.

Should there be a comma before kad pjeva u parku?

In this sentence, a comma is usually not written:

Njen glas je lijep kad pjeva u parku.

Time clauses with kad(a) placed at the end often appear without a comma in standard writing, unless you want a special pause or emphasis.
You would normally use a comma if the kad‑clause comes first:

Kad pjeva u parku, njen glas je lijep.

What is the difference between kad pjeva u parku and kad zapjeva u parku?

Both mean when she sings in the park, but there is a nuance in aspect:

pjeva (from pjevati) is imperfective – it focuses on the ongoing action or repeated/habitual singing: when she is singing / whenever she sings.
zapjeva (from zapjevati) is perfective – it focuses on the moment she starts to sing: when she starts to sing in the park.

So Njen glas je lijep kad pjeva u parku talks about her voice during the singing itself, not just the moment she begins.