Žao mi je što nove sandale ne stoje dobro uz kupaći kostim.

Žao mi je is the standard Croatian way to say I'm sorry.

Literally, it is closer to It is regrettable/sad to me:

• žao = sorry/regretful/sad
• mi = to me in the dative case
• je = is

So mi is there because Croatian expresses this idea as something that is felt to someone, not as I am sorry in the same structure English uses.

This is because both mi and je are short clitic forms, and Croatian has fairly strict clitic word order.

In this expression, the normal and fixed order is:

žao mi je

not žao je mi.

More generally, Croatian clitics tend to cluster near the beginning of the clause in a set order. For a learner, the safest thing is simply to memorize the whole expression as:

Žao mi je ... = I'm sorry ...

Što introduces the clause explaining what the speaker is sorry about.

So:

• Žao mi je što... = I'm sorry that...

Here it introduces: nove sandale ne stoje dobro uz kupaći kostim

After expressions of emotion like drago mi je, žao mi je, krivo mi je, Croatian very often uses što to mean that.

Because the adjective has to agree with the noun in gender, number, and case.

Here:

• sandale = feminine plural
• so nov- must also be feminine plural
• that gives nove

So:

• nova sandala = a new sandal
• nove sandale = new sandals

The form nove is not random; it matches sandale.

Because the subject is sandale, which is plural.

Croatian verbs agree with the subject in number (and in the past tense also gender).

So:

• sandala stoji = the sandal stands / looks / goes
• sandale stoje = the sandals stand / look / go

Since nove sandale is plural, the verb must be stoje.

Not literally, no.

The dictionary form is stajati, and one of its meanings is indeed to stand. But with clothing, colors, or items being combined, stajati can mean something like:

• to look good with
• to suit
• to go with
• to match

So in this sentence, ne stoje dobro uz... means that the sandals do not look good when paired with the swimsuit.

This is a very natural Croatian use of the verb.

Because dobro is an adverb, and it modifies the verb stoje.

It tells us how they go together:

• stoje dobro = they go well / they look good

If you used dobar/dobra/dobre, that would be an adjective, and adjectives describe nouns, not verbs.

So:

• dobre sandale = good sandals
• sandale dobro stoje uz... = the sandals go well with...

In this sentence, uz means something like:

• with
• together with
• paired with
• going with

So stajati dobro uz nešto is a common pattern for saying something matches something else visually.

Literally, uz often has meanings like next to, along, or up against, but in clothing/fashion contexts it often works like English with or go with.

It is in the accusative, because the preposition uz takes the accusative here.

So the object after uz should be accusative:

• uz kupaći kostim

Why does kostim look unchanged? Because kostim is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: kupaći kostim
• accusative: kupaći kostim

The case changed, but the form happened to stay the same.

Kupaći is an adjective meaning bathing or swimming-related.

So:

• kupaći kostim = bathing suit / swimsuit

You may also hear kupaći used on its own in everyday speech as a shortened way of saying swimsuit, but in this sentence it is functioning as an adjective modifying kostim.

Because in Croatian, ne is usually written separately from most verbs in the present tense.

So:

• stoje = they go / they stand
• ne stoje = they do not go / they do not stand

This is normal Croatian spelling.

A few common forms are written together, such as:

• nisam
• nemam
• neću

But with a regular present-tense verb like stoje, you write ne separately:

• ne stoje

Sometimes you may hear jer in speech, but with Žao mi je, što is the most natural and standard choice when introducing what someone is sorry about.

So:

• Žao mi je što kasnim.
• Žao mi je što nove sandale ne stoje dobro uz kupaći kostim.

Using što after žao mi je is something worth learning as a set pattern.

A rough guide for English speakers:

• ž = like the s in measure
  • Žao
• š = like sh in shoe
  • što
• ć = a soft sound somewhat like ty or a softer ch
  • kupaći

Also:

• j is pronounced like English y
  • je sounds roughly like ye

So:

• Žao mi je ≈ ZHAO mi ye
• što ≈ shto
• kupaći ≈ roughly koo-PA-chee, but with a softer ć

These are only approximations, but they are a useful starting point.