Learn Hardcore Croatian: Ljubazna gospođa sjedi na klupi u parku. - The kind lady is sitting on the bench in the park.

Croatian does not use articles at all, so there are no words that correspond directly to English a/an or the.
The noun gospođa can therefore mean a lady or the lady, depending on context.
If the speaker and listener already know which lady they’re talking about, you understand it as the lady; if not, as a lady.
To make something more specific, Croatian usually uses words like ta (that), ova (this), etc., e.g. Ta ljubazna gospođa = That kind lady / The kind lady.

Why does ljubazna come before gospođa, and why does it end in -a?

In Croatian, adjectives usually come before the noun they describe, so ljubazna gospođa is the normal order.
The ending -a on ljubazna shows that it is:

feminine
singular
in the nominative case

It has to agree with gospođa, which is also feminine singular nominative.
The base (dictionary) form of the adjective is ljubazan (masculine form), and it changes to ljubazna for a feminine noun like gospođa.

What is the difference between gospođa and žena?

gospođa is a polite word, similar to lady or the title Mrs.
- Ona je ljubazna gospođa. = She is a kind lady.
- Gospođa Novak = Mrs Novak
žena literally means woman, and also wife in many contexts.
- Ona je žena. = She is a woman.
- Moja žena = my wife

So in Ljubazna gospođa sjedi…, gospođa sounds polite and respectful, not just neutral “woman”.

What tense and form is sjedi, and why isn’t there a separate word for is?

Sjedi is the 3rd person singular present tense of the verb sjediti (to sit).
Croatian uses the present tense for both:

English sits (simple present) and
English is sitting (present continuous).

So Ljubazna gospođa sjedi can mean The kind lady sits or The kind lady is sitting, depending on context.
You do not say je sjedi; the verb form sjedi already includes the idea of is sitting.

How is the verb sjediti conjugated, and why is it sjedi here?

The infinitive is sjediti (to sit). In the present tense:

ja sjedim – I sit / am sitting
ti sjediš – you sit / are sitting (sg, informal)
on/ona/ono sjedi – he/she/it sits / is sitting
mi sjedimo – we sit / are sitting
vi sjedite – you sit / are sitting (pl/formal)
oni/one/ona sjede – they sit / are sitting

The subject ljubazna gospođa is she, so we use the 3rd person singular form: sjedi.

Why is it na klupi and not na klupa? What case is this?

Klupa is the basic form (nominative singular) meaning bench.
After the preposition na when it describes location (where something is), Croatian typically uses the locative case.

The locative singular of klupa is klupi, so we say:

na klupi = on (the) bench

If there was movement onto the bench (change of place), you would use the accusative instead:

Sjedne na klupu. = She sits down on the bench. (movement onto)

Why is it u parku and not u park? Is this the same case as na klupi?

Yes. Park is the nominative singular form, but after u for location (inside something), Croatian also uses the locative case.

Locative singular of park is parku, so:

u parku = in (the) park

Again, with movement into the park, you’d use the accusative:

Ulazi u park. = He/She is entering the park.

What is the difference between u and na in this sentence?

Very roughly:

u usually means in / inside
na usually means on / onto / at

So here:

na klupi – she is on a bench (on its surface)
u parku – the bench (and she) is in the park, inside that area

Many uses of u and na are straightforward, but there are also idiomatic patterns (e.g. u školi = at school, na poslu = at work) that you learn over time.

Can I change the word order, for example Gospođa ljubazna sjedi na klupi u parku? What would that sound like?

The neutral, most natural word order is Ljubazna gospođa sjedi na klupi u parku.
You can say Gospođa ljubazna sjedi…, but it sounds marked – a bit poetic, literary, or emphasizing ljubazna in a special way.

In normal speech and writing, keep the adjective before the noun:

ljubazna gospođa, stari čovjek, velika kuća, etc.

How would the sentence change if it was a man instead of a lady?

You would use the masculine noun and make the adjective masculine to match:

Ljubazan gospodin sjedi na klupi u parku.
- ljubazan – masculine form of ljubazan/ljubazna/ljubazno
- gospodin – gentleman / Mr.

The verb sjedi, and the rest of the sentence (na klupi u parku), stay the same, because they don’t depend on gender here.

How do you pronounce special combinations like lj, đ and sj in this sentence?

Approximate English-like pronunciations:

lj in Ljubazna: similar to the lli in million pronounced together – a soft ly sound: LYU-baz-na.
đ in gospođa: like j in judge: GOS-po-ja (closer: GOS-po-dja).
sj in sjedi: pronounced s
- j (like y), so roughly SYE-di.

Stress in standard Croatian is usually on the first syllable of each word (details are more complex, but “first syllable stress” is a good start):
LJUbazna GOSpođa SJEd i na KLUpi u PARku.

Does Ljubazna gospođa sjedi na klupi u parku sound complete by itself, or is anything missing in Croatian?

It is a fully natural, complete sentence in Croatian.
You don’t need any extra verb (like is) or any article (a/the).
In context, it can describe what you are seeing right now (“A kind lady is sitting on a bench in the park”) or a general fact, depending on how it’s used.