Learn Icelandic: Þetta er mjög dularfullur sjúkdómur.

The suffix -dómur creates masculine abstract nouns, usually indicating a state, condition, or domain. Here, it is added to the root of the adjective sjúkur (sick). Together, it forms sjúkdómur, literally meaning the condition of being sick, which translates to a disease.

Why do we use the neuter pronoun þetta (this) when the noun sjúkdómur is masculine?

When introducing or identifying something with 'This is...', Icelandic uses the neuter pronoun þetta as a fixed dummy subject, regardless of the gender of the noun being introduced. However, notice that the adjective dularfullur (mysterious) still correctly takes the masculine nominative ending -ur to agree with the masculine noun sjúkdómur.

How is the adjective dularfullur constructed?

It is made of the noun dul (secrecy, concealment) and the common suffix -fullur (full of). When combining them, the first word takes its genitive singular form, dular-, making the literal meaning 'full of secrecy'.

How would the phrase change if we were talking about 'the' very mysterious disease?

When you add the definite suffix to the noun to make it sjúkdómurinn, the preceding adjective must change to its weak declension. For a masculine nominative adjective, that means dropping the -ur and adding -i, resulting in the phrase mjög dularfulli sjúkdómurinn.