Etterkontrollen er viktig.

Because Norwegian usually writes compound nouns as a single word.

Here, etterkontrollen is built from:

• etter = after / follow-up
• kontroll = check, inspection, control
• -en = the definite ending

So etterkontrollen is one compound noun meaning something like the follow-up inspection/check.

If you split it up, the meaning changes. For example:

• etterkontrollen = the follow-up inspection
• etter kontrollen = after the inspection

That is a very important difference in Norwegian.

-en is the definite singular ending for many masculine/common-gender nouns in Bokmål. It often corresponds to English the.

So:

• en etterkontroll = a follow-up inspection
• etterkontrollen = the follow-up inspection

Norwegian often marks definiteness by attaching it to the noun, instead of using a separate word like English the.

Because in Norwegian, the noun often carries the definite meaning by itself.

So you normally say:

• etterkontrollen = the follow-up inspection

not:

• den etterkontrollen in a neutral sentence

You usually use den when there is an adjective before the noun, or when you want extra emphasis. For example:

• den viktige etterkontrollen = the important follow-up inspection

So in Etterkontrollen er viktig, the ending -en already gives the meaning the.

The base form is etterkontroll.

You could think of the forms like this:

• en etterkontroll = indefinite singular
• etterkontrollen = definite singular
• etterkontroller = indefinite plural
• etterkontrollene = definite plural

The gender of the whole compound comes from the last part, kontroll, so the compound behaves like en kontroll.

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

So:

• å være = to be
• er = is / am / are depending on context

In this sentence:

• Etterkontrollen er viktig = The follow-up inspection is important

Norwegian does not change the present-tense form of å være for different persons the way English does. So er works with many different subjects.

Here, viktig is a predicative adjective, meaning it comes after er and describes the subject.

In Norwegian, predicative adjectives agree mainly for gender and number, but they do not take the definite form just because the noun is definite.

So:

• Etterkontrollen er viktig = singular → viktig
• Etterkontrollene er viktige = plural → viktige

Compare that with an adjective placed before the noun:

• en viktig etterkontroll = an important follow-up inspection
• den viktige etterkontrollen = the important follow-up inspection

So even though etterkontrollen is definite, viktig stays in the normal singular form because it comes after er.

Yes. This is the standard word order for a basic statement:

• subject + verb + adjective/complement

So:

• Etterkontrollen = subject
• er = verb
• viktig = complement

A yes/no question would switch the order:

• Er etterkontrollen viktig? = Is the follow-up inspection important?

That subject-verb inversion is very common in Norwegian questions.

Yes, but it means something slightly different.

• Etterkontrollen er viktig = The follow-up inspection is important
  This refers to a specific follow-up inspection.

• En etterkontroll er viktig = A follow-up inspection is important
  This is more general.

So the difference is the same kind of difference as the versus a in English.

In the plural, you would change both the noun and the adjective:

• Etterkontroller er viktige = Follow-up inspections are important
• Etterkontrollene er viktige = The follow-up inspections are important

Notice that the adjective becomes viktige in the plural.

A helpful learner-level approximation is:

ET-ter-kon-TROL-len

A few things to notice:

• The tt makes the preceding vowel short.
• The r in er is part of the first element etter.
• The definite ending -en is usually unstressed.

In compounds, the first part often gets strong stress, but the second part can still sound prominent too, especially in a longer word like this one. The most important thing for a learner is to keep it as one connected word, not two separate words.