Pokrivenost mrežom i brzina interneta su važne.

Because the subject contains two things joined by i:

• pokrivenost mrežom
• brzina interneta

Together, they form a plural subject, so Croatian uses plural agreement:

• su = are
• važne = important in the plural

If you mentioned only one item, you would use singular:

• Pokrivenost mrežom je važna.
• Brzina interneta je važna.

Both nouns in the subject are feminine:

• pokrivenost = feminine
• brzina = feminine

So the predicate adjective važan must agree with them in gender and number. Since there are two feminine nouns, the form is važne.

A useful comparison:

• masculine plural: važni
• feminine plural: važne
• neuter plural: važna

If the nouns were of mixed gender, Croatian often uses masculine plural as the default.

Yes. Many Croatian nouns ending in -ost are feminine, even though they do not end in -a.

Common examples:

• mogućnost = possibility
• sigurnost = safety
• vrijednost = value
• brzost is not common, but brzina is feminine too

So pokrivenost is feminine, and in the singular you would say:

• Pokrivenost mrežom je važna.

Mrežom is the instrumental singular of mreža.

• nominative: mreža
• instrumental: mrežom

Here, instrumental is used in a common expression meaning something like coverage by means of a network or network coverage. In telecom language, pokrivenost mrežom is very natural.

So even though it may look unusual at first, it is a normal Croatian way to express the idea.

Interneta is the genitive singular of internet.

• nominative: internet
• genitive: interneta

After many nouns, Croatian uses the genitive to show a relationship similar to English of or an English noun-noun phrase.

So:

• brzina interneta = speed of the internet = internet speed

This is very common in Croatian. Compare:

• cijena karte = ticket price
• boja auta = car color
• kvaliteta usluge = service quality

Because pokrivenost mrežom and pokrivenost mreže do not mean the same thing.

• pokrivenost mrežom = coverage provided by a network, the usual telecom meaning
• pokrivenost mreže would sound more like the coverage of the network as a property of the network itself, and is not the normal phrase here

So in this sentence, mrežom is the natural choice.

Croatian does not have articles like English a and the.

That means nouns appear without articles, and definiteness is understood from context.

So:

• brzina interneta can mean internet speed or the speed of the internet
• pokrivenost mrežom can mean network coverage

This is completely normal in Croatian.

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

The original sentence is neutral and natural:

• Pokrivenost mrežom i brzina interneta su važne.

But you could also say:

• Brzina interneta i pokrivenost mrežom su važne.
• Važne su pokrivenost mrežom i brzina interneta.

These versions may shift emphasis slightly, but the basic meaning stays the same.

It is a very common collocation, especially in telecom and mobile-service contexts.

You will often see similar phrases such as:

• pokrivenost signalom = signal coverage
• pokrivenost 5G mrežom = 5G network coverage
• loša pokrivenost mrežom = poor network coverage

So it is worth learning pokrivenost + instrumental as a useful pattern.

Then everything would become singular.

For example:

• Pokrivenost mrežom je važna.
• Brzina interneta je važna.

This is a good way to see the agreement clearly:

• one thing: je važna
• two things: su važne

Yes, that is a very good way to understand it.

English often uses one noun directly before another:

• internet speed
• car door
• service quality

Croatian often expresses the same idea with a noun followed by a genitive noun:

• brzina interneta
• vrata auta
• kvaliteta usluge

So brzina interneta is a very typical Croatian structure.