Góður svefn er nauðsynlegur, en annað kaffi er óþarfi núna.

Because Icelandic adjectives must agree with the noun they describe in gender, number, and case.

• svefn is masculine singular
• kaffi is neuter singular

So the adjective changes form:

• góður svefn = good sleep
• annað kaffi = another coffee

The adjective annar is a bit irregular, and its neuter singular form is annað.

• svefn = masculine
• kaffi = neuter

You need to know the gender because it affects:

• adjective endings
• pronouns
• sometimes article forms

That is why you get:

• góður svefn
• annað kaffi

instead of using one adjective form for everything, as in English.

Both main noun phrases are in the nominative singular:

• Góður svefn
• annað kaffi

They are the subjects of the verb er (is), so nominative is the normal case.

The predicate adjective in the first clause, nauðsynlegur, also matches the subject in nominative masculine singular.

Because nauðsynlegur describes the subject svefn after the verb er. In Icelandic, predicate adjectives usually agree with the subject.

So:

• Góður svefn = masculine singular nominative
• nauðsynlegur = also masculine singular nominative

That is why both adjectives end in -ur here.

This is a very common learner question.

In this sentence, óþarfi is best understood as a noun-like expression meaning something unnecessary or a needless thing. Icelandic very often says:

• X er óþarfi = X is unnecessary

So literally, the second clause is close to:

• Another coffee is a needless thing now

which English naturally translates as:

• Another coffee is unnecessary now

If you used the adjective óþarfur directly, the neuter form would be óþarft, but er óþarfi is a very normal and idiomatic way to say this.

Here annað kaffi means another coffee, usually in the sense of one more cup of coffee.

It does not usually mean a different type of coffee in this kind of sentence. The idea is more like:

• a second coffee
• one more coffee

Context decides the exact nuance, but that is the most natural reading here.

Because Icelandic has no indefinite article.

English says:

• a good sleep
• a coffee

Icelandic simply uses the noun without an indefinite article:

• góður svefn
• kaffi

If Icelandic wants to express the, it usually adds the definite article as a suffix to the noun.

Because there are two separate clauses joined by en (but):

1. Góður svefn er nauðsynlegur
2. annað kaffi er óþarfi núna

Each clause needs its own finite verb, so er appears in both halves.

This works very much like English:

• Good sleep is necessary, but another coffee is unnecessary now.

En means but. It connects two contrasting ideas:

• good sleep is necessary
• another coffee is unnecessary now

So it marks a contrast between what is needed and what is not needed.

Núna means now or right now.

It makes the statement more temporary and situation-specific:

• another coffee is unnecessary now

Without núna, the sentence would sound more general. With núna, it suggests that at this moment, another coffee is not needed.

They are strong forms.

That is why you see forms like:

• góður
• nauðsynlegur
• annað

Icelandic generally uses strong adjective forms when there is no definite article, possessive, or demonstrative before the noun.

Very roughly:

• góður svefn = good sleep
• compare weak forms in definite contexts, such as góði... in other kinds of phrases

So yes, this sentence is a good example of normal strong adjective agreement.

No, this is very straightforward Icelandic word order.

Each clause follows a basic pattern:

• subject + er + complement

So:

• Góður svefn
  • er
    • nauðsynlegur
• annað kaffi
  • er
    • óþarfi
      • núna

For a learner, this sentence is nice because the structure is quite close to English.