Breakdown of Það er dimmt úti og rigningin er köld.
vera
to be
og
and
það
it / there (existential) / the one (n)
úti
outside
Questions & Answers about Það er dimmt úti og rigningin er köld.
Since we are just talking about the weather, why do we need the word Það?
Það translates to "It" and acts as a dummy subject here. Just like in English, Icelandic sentences usually need a subject. When describing the weather or general conditions where nobody is actually doing an action, we use Það to fill that empty subject spot.
Why does the first adjective get a -t ending (dimmt) but the second one doesn't (köld)?
Adjectives in Icelandic change to match the gender of the noun they describe. In the first half of the sentence, dimmt describes the dummy subject það, which is a neuter word. In the second half, köld describes rigningin (the rain), which is a feminine noun.
How does rigningin mean "the rain"?
In Icelandic, "the" is not a separate word that goes before the noun. Instead, it gets attached to the end of the noun as a suffix. The base word is rigning (rain), and the -in at the end is the feminine definite article, making it rigningin (the rain).
Could I use út instead of úti for "outside" here?
No, because Icelandic uses different words depending on whether you mean location or movement. Úti means you are already located outside, which fits this sentence perfectly because it describes a current state. You would only use út if there was movement involved, like walking outside.