Ljubičasta haljina dobro joj stoji.

Because ljubičasta haljina is the subject of the sentence, so it is in the nominative case.

The adjective ljubičasta has to agree with haljina in:

• gender: feminine
• number: singular
• case: nominative

So:

• ljubičasta haljina = nominative singular
• ljubičastu haljinu = accusative singular

You would use ljubičastu haljinu in a sentence like Vidim ljubičastu haljinu.

It is nominative singular.

In this sentence, the thing doing the grammatical action is the purple dress, even though in English we often think of her as the important person. Croatian treats this pattern as:

• the dress = nominative
• to her = dative

So the structure is basically:

[Item of clothing] + [to someone, dative] + [looks good / suits]

Joj is the dative singular clitic form of ona.

Here it means to her.

With stajati in the sense of to suit / to look good on someone, Croatian uses the person in the dative:

• Haljina joj stoji. = The dress suits her.
• Kaput mu stoji. = The coat suits him.
• To mi ne stoji. = That does not suit me.

So joj is there because Croatian says something like The dress stands well to her.

Yes, stoji literally comes from stajati, which usually means to stand.

But with clothes, colors, hairstyles, and similar things, stajati often means:

• to suit
• to look good on
• to be becoming

So here stoji does not mean that the dress is physically standing somewhere. It means the dress looks good on her.

This is a very common Croatian use:

• Ta boja ti dobro stoji. = That color suits you.
• Kratka kosa joj lijepo stoji. = Short hair looks nice on her.

Usually not primarily.

In this kind of sentence, dobro joj stoji usually means:

• it looks good on her
• it suits her appearance
• it is becoming

It can sometimes imply that the fit is good, but the main idea is usually appearance, not exact size.

If you specifically want to talk about size or physical fit, Croatian might use other expressions depending on context.

Dobro is an adverb meaning well.

It modifies the verb stoji, not the noun haljina.

So:

• dobro stoji = stands well / suits well / looks good

This is why Croatian uses dobro, not an adjective form matching haljina.

Compare:

• dobra haljina = a good dress
• haljina dobro stoji = the dress looks good on someone

Yes, Croatian word order is fairly flexible, but joj is a clitic, so it cannot just go anywhere.

The given sentence is natural:

• Ljubičasta haljina dobro joj stoji.

You may also hear:

• Ljubičasta haljina joj dobro stoji.
• Dobro joj stoji ljubičasta haljina.

The difference is mostly about emphasis and style, not basic meaning.

A useful thing to remember is that joj is an unstressed short form, so it tends to stay in a restricted position and does not normally carry emphasis by itself.

Usually, no — not in a neutral sentence.

• joj = unstressed clitic form
• njoj = stressed full form

In an ordinary sentence, Croatian normally uses joj:

• Ljubičasta haljina dobro joj stoji.

You use njoj when you want contrast or special emphasis:

• Ljubičasta haljina dobro stoji njoj, ali ne sestri.

So if you are just saying The purple dress suits her well, joj is the normal choice.

Because Croatian does not have articles like English a/an/the.

So ljubičasta haljina can mean:

• a purple dress
• the purple dress

Which one is meant depends on context.

If Croatian speakers want to make it more specific, they can use things like:

• ta ljubičasta haljina = that purple dress
• ova ljubičasta haljina = this purple dress

Because it agrees with haljina, which is singular.

• haljina = singular
• stoji = 3rd person singular present

If the subject were plural, the verb would change:

• Ljubičaste cipele dobro joj stoje. = The purple shoes suit her well.

So the verb agrees with the item of clothing, not with joj.

Yes, very common.

Croatian often uses stajati for things that suit someone visually:

• clothes
• colors
• hairstyles
• glasses
• makeup styles

Examples:

• Crvena ti dobro stoji. = Red suits you.
• Ta košulja mu lijepo stoji. = That shirt looks good on him.
• Duga kosa joj ne stoji baš najbolje. = Long hair does not suit her very well.

So this sentence is a very useful model to remember.

You mainly change the dative pronoun:

• Ljubičasta haljina mi dobro stoji. = The purple dress suits me well.
• Ljubičasta haljina ti dobro stoji. = The purple dress suits you well.
• Ljubičasta haljina mu dobro stoji. = The purple dress suits him well.
• Ljubičasta haljina nam dobro stoji. = The purple dress suits us well.
• Ljubičasta haljina vam dobro stoji. = The purple dress suits you well.
• Ljubičasta haljina im dobro stoji. = The purple dress suits them well.

So a very helpful pattern is:

[subject in nominative] + dobro + [dative pronoun] + stoji