Sængin er hlý, en lakið er enn kalt.

They are two different words:

• en = but
• enn = still, yet, or sometimes even depending on context

So in this sentence:

• Sængin er hlý = the duvet/blanket is warm
• en = but
• lakið er enn kalt = the sheet is still cold

The extra n matters a lot.

Because Icelandic often puts the definite article directly onto the end of the noun instead of using a separate word like English the.

Here:

• sængin = sæng
  • -in = the duvet/blanket
• lakið = lak
  • -ið = the sheet

So English the is built into the noun.

Because the nouns have different grammatical genders.

• sæng is feminine
• lak is neuter

In the nominative singular definite form, the article ending changes depending on gender:

• feminine often uses -in
• neuter often uses -ið

So:

• sæng → sængin
• lak → lakið

That i in lakið is a normal linking vowel in the definite form.

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

Here both nouns are singular and nominative, but:

• sængin is feminine, so the adjective is hlý
• lakið is neuter, so the adjective is kalt

So the adjective changes form to match the noun it describes.

Because these are predicate adjectives: they come after the verb vera = to be.

In Icelandic, adjectives used after vera usually appear in the regular agreeing form, not the weak definite form you often see before a definite noun.

So:

• sængin er hlý = the duvet is warm
• lakið er kalt = the sheet is cold

But when the adjective comes directly before a definite noun, weak forms are common:

• kalda lakið = the cold sheet

So the sentence uses predicate adjective forms, not attributive weak forms.

Er is the 3rd person singular present form of vera = to be.

So:

• sængin er hlý = the duvet is warm
• lakið er enn kalt = the sheet is still cold

It appears twice because there are two clauses joined by en:

• clause 1: Sængin er hlý
• clause 2: lakið er enn kalt

Each clause needs its own finite verb.

They are in the nominative singular.

Why?

• sængin and lakið are the subjects of their clauses
• after vera, the predicate adjective normally agrees with the subject

So:

• sængin → nominative singular feminine
• hlý → nominative singular feminine
• lakið → nominative singular neuter
• kalt → nominative singular neuter

Because enn is an adverb modifying the adjective phrase, and in a neutral Icelandic sentence it normally comes after the verb and before the adjective.

So:

• lakið er enn kalt = the sheet is still cold

This is the most natural basic order.
A useful pattern to remember is:

• subject + verb + adverb + adjective

Here that gives:

• lakið
  • er
    • enn
      • kalt