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