Lykillinn er fastur, svo ég get ekki opnað hurðina.

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Breakdown of Lykillinn er fastur, svo ég get ekki opnað hurðina.

• vera: to be
• ekki: not
• opna: to open

Questions & Answers about Lykillinn er fastur, svo ég get ekki opnað hurðina.

Why is it Lykillinn and not just lykill?

Lykillinn is lykill (key) with the definite article attached: -inn = the.

• lykill = a key / key
• lykillinn = the key

Icelandic usually puts the as a suffix on the noun (instead of a separate word like English).

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What case is Lykillinn in, and how do I know?

It’s nominative singular because it’s the subject of the sentence (the thing that is something):
Lykillinn er fastur = The key is stuck.
With the verb vera (to be), the subject stays in the nominative.

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Why is the adjective fastur ending in -ur?

Because adjectives agree with the noun they describe in gender, number, and case.
lykill is masculine, singular, nominative, so the adjective takes the masculine nominative singular form:

• masculine nom. sg.: fastur
  (You’ll see different forms with other genders/cases, e.g. föst / fast / föstu, depending on context.)

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Does fastur mean fixed or stuck here?