U parku su nova klackalica i visoka penjalica.

Because the sentence has a compound subject: nova klackalica i visoka penjalica = a new seesaw and a tall climbing frame. Even though each noun is singular, together they make a plural idea, so Croatian uses the plural form su.

• je = is
• su = are

So Croatian treats this as Two things are in the park, not One thing is in the park.

Because each adjective describes just one noun:

• nova goes with klackalica
• visoka goes with penjalica

Both nouns are singular feminine, so each adjective is also singular feminine.

This is different from the verb. The verb agrees with the whole subject taken together, while each adjective agrees only with its own noun.

Here u means in, showing location, so it takes the locative case.

• park = nominative/basic form
• u parku = in the park (locative)

So u parku is the correct form for being somewhere.

Yes. With movement, u takes the accusative, not the locative.

Compare:

• u parku = in the park / located in the park
• u park = into the park / movement toward the inside of the park

For example:

• Djeca su u parku. = The children are in the park.
• Djeca idu u park. = The children are going to the park.

So the case changes depending on whether the sentence describes location or movement.

They are in the nominative case because they are the subject of the sentence.

Even though they come after the verb, they are still the things that are in the park. Croatian word order is flexible, so the subject does not always have to come first.

Croatian often puts the location first to set the scene. It is very natural to begin with where something is, and then say what is there.

So:

• U parku su nova klackalica i visoka penjalica.

is natural and means something like:

• In the park, there are a new seesaw and a tall climbing frame.

You could also say:

• Nova klackalica i visoka penjalica su u parku.

That is also grammatical, but the emphasis is a little different.

There usually is no separate word for that. Croatian does not need a dummy subject like English there.

English says:

• There are two things in the park.

Croatian simply says:

• U parku su dvije stvari.

So in your sentence, U parku su... naturally carries the idea of There are... in the park.

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

So nova klackalica can mean:

• a new seesaw
• the new seesaw

The exact meaning depends on context. In many basic examples, English translations use a because that sounds most natural, but Croatian itself does not mark that difference with an article.

They are feminine nouns, and one strong clue is that they end in -a, which is very common for feminine nouns in Croatian.

Because they are feminine singular, the adjectives must also be feminine singular:

• nova klackalica
• visoka penjalica

That is why you see nova and visoka, not masculine forms like novi or visoki.

Yes. In ordinary Croatian, descriptive adjectives usually come before the noun:

• nova klackalica
• visoka penjalica

That is the normal, neutral order. Croatian can sometimes move words around for style or emphasis, but adjective + noun is the standard pattern learners should use first.