Learn Hardcore Icelandic: Hitastigið breytist hratt.

Hitastig is a neuter noun formed from hita- (“heat”) + -stig (“level”). In Icelandic you add -ið to a neuter noun in the nominative singular to make it definite (“the temperature” rather than “a temperature”). So hitastig = “temperature” (indefinite), hitastigið = “the temperature.”

What is the difference between breytist and breytir?

Breyta is a transitive (active) verb meaning “to change something” and takes a direct object in the dative case with the ending -ir in 3rd person singular (breytir). Breyta used in the middle/intransitive voice becomes breytast, meaning “to change” by itself (“is changing”). That’s why when the temperature changes on its own, you use breytist (3rd person sing. of breytast).

Why is the verb form breytist and what is its pattern?

Breyta belongs to the ST-verb group, which in the middle/intransitive voice adds -st in the present tense. So the pattern is:
• Active/transitive: breytir
• Middle/intransitive: breytist
Here breytist is 3rd person singular present of the intransitive (middle) form.

Why is hratt used instead of hraður or hraðar?