Vitenskapen er viktig.

Because Norwegian often uses the definite form when talking about a field, institution, or concept as a whole. So vitenskapen can mean science in a general sense here, even though English does not use the in Science is important.

The bare form vitenskap can exist too, but vitenskapen sounds very natural when referring to science as a whole.

-en is the definite singular ending for a common-gender noun in Bokmål.

So:

• vitenskap = science / a science (dictionary form)
• en vitenskap = a science
• vitenskapen = the science / science

Unlike English, Norwegian usually attaches the to the end of the noun.

Er is the present tense of å være, which means to be.

So:

• å være = to be
• er = am / is / are

Norwegian does not change the verb for different persons the way English does. For example:

• jeg er = I am
• du er = you are
• han er = he is
• de er = they are

Because viktig is a predicate adjective here: it comes after er and describes the subject.

In this kind of sentence, a singular subject normally takes the singular adjective form, and for viktig, that form is viktig.

Compare:

• Vitenskapen er viktig. = Science is important.
• Vitenskapene er viktige. = The sciences are important.

Also note that viktig does not change in the neuter singular either:

• et viktig spørsmål = an important question

So viktig is the expected form here.

Vitenskap is normally a common-gender noun in Bokmål.

That is why you get:

• en vitenskap
• vitenskapen

and not:

• et vitenskap

Knowing the gender matters because it affects articles and some other forms in Norwegian.

In this sentence, it most naturally means science in general.

If you wanted to talk about one specific science or discipline, you would usually make that clearer, for example:

• en vitenskap = a science
• denne vitenskapen = this science
• den vitenskapen = that science

So here the sentence is best understood as a general statement about science as a whole.

This is the basic Norwegian statement pattern:

• Subject + verb + complement

So:

• Vitenskapen = subject
• er = verb
• viktig = complement/adjective

The sentence is very straightforward.

A useful extra point: Norwegian is a verb-second language, so if you put something else first, the verb still stays in second position:

• I dag er vitenskapen viktig. = Today, science is important.

That is a bigger rule in Norwegian word order.