Mislim da ta plava suknja dobro stoji uz bijelu košulju.

Because mislim da ... is the normal Croatian way to say I think that ... or simply I think ....

• Mislim = I think
• da introduces the following clause

So:

• Mislim da ta plava suknja dobro stoji uz bijelu košulju. = I think that that blue skirt goes well with the white shirt.

In Croatian, da is extremely common after verbs of thinking, knowing, saying, believing, etc.

Suknja is in the nominative singular because it is the subject of the clause.

The core statement is:

• ta plava suknja dobro stoji uz bijelu košulju

Here, ta plava suknja is the thing that goes well with something else, so it is the subject.

Because ta, plava, and suknja all have to agree with each other in:

• gender
• number
• case

Here the noun suknja is:

• feminine
• singular
• nominative

So both the demonstrative and the adjective must match:

• ta = feminine singular nominative of that
• plava = feminine singular nominative of blue
• suknja = skirt

That is standard Croatian agreement.

Because uz takes the accusative case here, so košulja changes to košulju, and the adjective must change too:

• bijela košulja = nominative
• bijelu košulju = accusative

So after uz, you get:

• uz bijelu košulju

This is one of the most important things to notice in the sentence: the second noun phrase is not in the nominative.

In ordinary use, yes, uz is followed by the accusative.

Its basic meanings include things like:

• next to
• along
• by
• sometimes with, in the sense of pairing or matching

In this sentence, uz is part of the expression stajati uz in the sense of to go with / to match.

So:

• uz bijelu košulju = with the white shirt / together with the white shirt

Literally, stajati means to stand, and stoji is stands.

But in clothing and style contexts, dobro stoji uz is idiomatic and means:

• goes well with
• matches
• looks good with

So here it is not about physical position. The skirt is not literally standing next to the shirt. It is about visual matching.

Because dobro is an adverb, not an adjective.

It modifies the verb phrase stoji uz, meaning:

• it goes well with
• it matches well

Compare:

• dobra suknja = a good skirt
  Here dobra is an adjective describing the noun.

• suknja dobro stoji uz košulju = the skirt goes well with the shirt
  Here dobro is an adverb describing how well it goes with it.

Yes. If you say:

• Mislim da plava suknja dobro stoji uz bijelu košulju

that still works.

The difference is that ta means that, so it points to a specific skirt, probably one already known in the conversation or physically visible.

• ta plava suknja = that blue skirt
• plava suknja = a/the blue skirt, depending on context

Since Croatian has no articles like the and a, words like ta often help make something more specific.

Because in Croatian, descriptive adjectives normally come before the noun.

So:

• plava suknja = blue skirt
• bijela košulja = white shirt

This is the most neutral and common order. Croatian word order is flexible in some ways, but adjective + noun is the standard pattern here.

Yes, Croatian word order is fairly flexible, but not every version sounds equally natural in every context.

The given sentence:

• Mislim da ta plava suknja dobro stoji uz bijelu košulju.

is very natural.

You could also say:

• Ta plava suknja, mislim, dobro stoji uz bijelu košulju.

But that changes the rhythm and emphasis.

Starting with Mislim da ... sounds like a normal opinion statement: I think that ...

Stoji is the 3rd person singular present tense of stajati.

So:

• ja stojim = I stand
• ti stojiš = you stand
• on/ona/ono stoji = he/she/it stands

In this sentence, the subject is ta plava suknja, which is singular, so the verb must also be singular:

• suknja stoji

Yes. It is the expected feminine singular accusative form of bijel / bijela.

You can think of the basic feminine nominative form as:

• bijela

and after uz it becomes accusative:

• bijelu

So:

• bijela košulja = nominative
• vidim bijelu košulju = accusative
• uz bijelu košulju = accusative

This kind of adjective ending change is completely normal in Croatian.

Yes. Croatian also often uses ići uz in this kind of context.

For example:

• Ta plava suknja dobro ide uz bijelu košulju.

That also means That blue skirt goes well with the white shirt.

A small nuance:

• dobro stoji can suggest that something looks good in combination, sometimes with a slightly more visual or style-related feel
• dobro ide uz is also very common for matching and pairing

Both are natural.

A helpful breakdown is:

• Mislim = I think
• da = that
• ta plava suknja = that blue skirt
• dobro stoji uz = goes well with
• bijelu košulju = the white shirt

So the grammar skeleton is:

• [I think] + [that-clause]
• [subject] + [adverb] + [verb + preposition] + [object in accusative]

That is a very useful pattern you can reuse with many other sentences.