Learn Icelandic: Dulmálið var auðséð. - The cipher was easily seen.

The prefix auð- means "easily" or "easy to". It is attached directly to the past participle of a verb to form a new adjective. Here, it attaches to séð, which is the past participle of sjá (to see), creating a word that means "easily seen" or "obvious".

Why does the word end in -séð instead of the dictionary form -sénn?

Adjectives formed with auð- and tor- must agree with the noun they describe. Because Dulmálið is a neuter noun, the adjective takes the neuter singular nominative form auðséð. If you were describing a masculine noun, like textinn (the text), it would be auðsénn.

How is the noun Dulmálið built?

It is a compound word made of dul (meaning "secrecy" or "concealment") and mál (meaning "language"). Together they mean "cipher" or "secret language". The -ið at the end is the attached definite article, making it "the cipher".

If the cipher was hard to see, how would I use the prefix tor- here?

You would simply replace the prefix and say Dulmálið var torséð. The prefix tor- acts exactly like auð- but means "with difficulty" or "hard to", so torséð means "hard to see".