Kontakt s policijom je važan.

Because the preposition s normally requires the instrumental case in Croatian when it means with.

• base form: policija = police
• instrumental singular: policijom

So:

• s policijom = with the police

This is a very common pattern:

• s prijateljem = with a friend
• s obitelji = with the family
• s učiteljem = with the teacher

In this sentence, kontakt s policijom literally means contact with the police.

Kontakt is in the nominative singular because it is the subject of the sentence.

You can identify it by asking: What is important?

Answer: Kontakt s policijom.

So the structure is:

• Kontakt s policijom = subject
• je = is
• važan = important

Because važan has to agree with kontakt, and kontakt is a masculine singular noun.

Croatian adjectives agree with the noun in:

• gender
• number
• case

Here:

• kontakt = masculine singular
• so the adjective is važan

Compare:

• kontakt je važan = contact is important
• situacija je važna = the situation is important
• pitanje je važno = the question is important

Je is the 3rd person singular present tense of the verb biti = to be.

So:

• je = is

In this sentence:

• Kontakt s policijom je važan = Contact with the police is important

Croatian often uses forms of biti just like English uses am / is / are.

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

So kontakt can mean:

• contact
• a contact
• the contact

And policijom can mean:

• police
• the police

The exact meaning depends on context. That is completely normal in Croatian.

Normally, the standard form here is s policijom.

Croatian has both s and sa, but sa is usually used:

• before certain difficult consonant combinations
• before words beginning with s, š, z, or similar sounds
• in some fixed expressions

For example:

• sa sestrom
• sa školom
• sa mnom

But with policijom, the usual form is s policijom.

In Croatian, policija is a singular feminine noun.

So even though English uses police, Croatian treats policija as one singular noun:

• policija je stigla = the police arrived

In your sentence, the form is instrumental singular:

• policija → policijom

So do not think of it as an English-style plural. Grammatically, it is singular in Croatian.

Yes. Croatian word order is more flexible than English word order.

The neutral order here is:

• Kontakt s policijom je važan.

But you may also hear:

• Važan je kontakt s policijom.

This puts more emphasis on važan = important.

Both are grammatical, but the original sentence is a very natural, neutral way to say it.

Because in this sentence važan is a predicate adjective: it describes the subject after the verb to be.

So the pattern is:

• subject + je + adjective

Examples:

• Grad je velik. = The city is big.
• Problem je ozbiljan. = The problem is serious.
• Kontakt s policijom je važan. = Contact with the police is important.

You can sometimes move the adjective for emphasis, but the basic pattern is very common.

It is a noun phrase made of:

• kontakt = the main noun
• s policijom = a prepositional phrase that explains what kind of contact

So literally:

• kontakt = contact
• s policijom = with the police

Together they function as one unit: contact with the police.

It is the normal masculine singular form used to match kontakt.

This is the form you will often see in dictionary-style examples:

• važan = important (masculine)
• važna = important (feminine)
• važno = important (neuter)

Since kontakt is masculine, važan is exactly the form you would expect.

If you made kontakt plural, the adjective would also become plural.

For example:

• Kontakti s policijom su važni. = Contacts with the police are important.

Changes:

• kontakt → kontakti
• je → su
• važan → važni

This shows again how Croatian adjectives and verbs agree with the noun.