Dekorasjonen er ferdig.

Because dekorasjonen is the definite singular form, meaning the decoration.

• en dekorasjon = a decoration
• dekorasjonen = the decoration

In Norwegian, definiteness is usually added to the end of the noun as a suffix:

• en bil = a car
• bilen = the car

So Dekorasjonen er ferdig literally matches The decoration is finished/ready.

The base form is dekorasjon.

This is a noun, and its common forms are:

• en dekorasjon = a decoration
• dekorasjonen = the decoration
• dekorasjoner = decorations
• dekorasjonene = the decorations

So in the sentence, you are seeing the noun in its definite singular form.

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

So:

• jeg er = I am
• du er = you are
• han/hun er = he/she is
• dekorasjonen er = the decoration is

In this sentence, er links the subject dekorasjonen with the description ferdig.

Ferdig often means finished, done, or ready, depending on context.

In this sentence, it can mean something like:

• finished
• complete
• ready

So Dekorasjonen er ferdig can suggest that the decoration has been completed and is now ready.

A few other examples:

• Maten er ferdig. = The food is ready / done.
• Jeg er ferdig. = I am finished / I’m done.

Because it is being used as a predicate adjective after er.

In Norwegian, adjectives often agree with the noun, but ferdig is one of those adjectives whose singular form looks the same in common gender and neuter.

Compare:

• Dekorasjonen er ferdig. = singular
• Huset er ferdig. = singular neuter
• Dekorasjonene er ferdige. = plural

So the important contrast is mainly:

• singular: ferdig
• plural / definite adjective contexts: ferdige

Because dekorasjonen is singular.

Use:

• ferdig with singular
• ferdige with plural

Examples:

• Dekorasjonen er ferdig. = The decoration is finished.
• Dekorasjonene er ferdige. = The decorations are finished.

So ferdige would be wrong here unless the subject were plural.

This sentence uses a very basic Norwegian word order:

Subject + verb + adjective

• Dekorasjonen = subject
• er = verb
• ferdig = adjective/complement

So it is very similar to English:

• The decoration
  • is
    • finished

Nothing unusual is happening here.

Usually, dekorasjonen refers more to the decoration itself rather than the act of decorating.

If you want to talk about the process, Norwegian would more naturally use something like:

• Dekoreringen er ferdig. = The decorating is finished.

So:

• dekorasjonen = the decoration, the decorated thing/result
• dekoreringen = the decorating, the process

Yes, sometimes, but klar and ferdig are not always exactly the same.

• ferdig emphasizes that something is finished/completed
• klar emphasizes that something is ready

In many contexts, both work:

• Dekorasjonen er ferdig.
• Dekorasjonen er klar.

But ferdig more strongly suggests that the work has been completed.

A rough pronunciation guide for an English speaker would be:

• Dekorasjonen ≈ deh-koh-rah-SHOO-nen
• er ≈ air or a short Norwegian e+r sound
• ferdig ≈ FEHR-dih

A few useful notes:

• The stress in dekorasjonen falls on -sjo-
• The sj sound in -sjon is like sh
• The final g in ferdig is not pronounced like a hard English g

A more approximate full pronunciation: deh-koh-rah-SHOO-nen er FEHR-dih

It is normally treated as a common gender noun in Bokmål, so you usually learn it as:

• en dekorasjon

Its definite singular form is:

• dekorasjonen

For this word, the forms you need most are simply:

• en dekorasjon
• dekorasjonen

No. In Norwegian, you normally need the verb er in a sentence like this.

So you say:

• Dekorasjonen er ferdig.

Not:

• Dekorasjonen ferdig.

Norwegian usually requires the verb to be in this kind of statement, just like English does.

You would change both the noun and the adjective:

• Dekorasjonene er ferdige. = The decorations are finished.

Changes:

• dekorasjonen → dekorasjonene
• ferdig → ferdige

That is a very common pattern in Norwegian when going from singular to plural.