Learn Icelandic: Lykilorðið gleymist alltaf. - The password always gets forgotten.

The active verb gleyma means "to forget" and requires someone doing the forgetting. By adding the middle voice -st ending to make gleymist, it translates to "gets forgotten" or "is forgotten". This lets you focus on the password itself without having to say exactly who is forgetting it.

The verb gleyma usually takes a dative object. Why is Lykilorðið in the nominative case?

This is a crucial rule for the middle voice! While you would use the dative in an active sentence (like Ég gleymdi lykilorðinu - I forgot the password), the middle voice turns the passive object into the subject of the new sentence. Subjects are almost always in the nominative case, so the dative lykilorðinu changes to the nominative lykilorðið.

What are the root words that make up Lykilorðið?

It is a compound of lykill (key) and orð (word), plus the neuter definite article -ið (the). When creating the compound, the first word drops its ending to use the stem lykil-.

If I wanted to emphasize "always", could I start the sentence with alltaf?

Yes, but you must remember the V2 (verb-second) rule. If you put Alltaf at the front, the verb must remain in the second slot. The sentence would become Alltaf gleymist lykilorðið.