Learn Icelandic: Mig langar að finna lykilinn.

The verb langa (to want or long for) is a special type of verb in Icelandic called an impersonal verb. Instead of taking a normal subject like ég (I), it forces the person who wants something into the accusative case, which is mig. It translates literally as "it longs me".

Why does the word for "the key" drop an "l", becoming lykilinn instead of lykillinn?

The verb finna (to find) requires the thing being found to be in the accusative case. The basic masculine noun for "key" is lykill, but its accusative form drops one "l" to become lykil. When we add the accusative definite article for "the" (-inn), the final word becomes lykilinn.

How can I tell that lykilinn is definite ("the key") and not just "a key"?

In Icelandic, "the" is attached to the end of the noun. If you just wanted to say "I want to find a key", you would leave off the definite ending and say Mig langar að finna lykil. Adding -inn to the accusative noun creates lykilinn (the key).

What is the role of að in this sentence?