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?

hraður is an adjective (“fast”) and must match a noun in gender, number, and case. hratt is the adverbial form (“quickly”) and modifies the verb breytist. In Icelandic many adverbs look like the neuter singular of the adjective.

Is the word order Hitastigið breytist hratt fixed, or could I swap elements?

Icelandic normally uses a V2 (verb-second) rule: the finite verb (breytist) must occupy the second position. If you start with the subject hitastigið, the order is exactly as given. You could front an adverb instead, e.g.: Hratt breytist hitastigið (“Quickly the temperature changes”). But you cannot put the verb first unless you omit the subject: Breyttist hratt hitastigið would sound odd.

How do I turn this into a yes/no question?

Invert the subject and the verb (still keeping V2):
Breyttist hitastigið hratt?
This means “Is the temperature changing quickly?”

Can I talk about multiple temperatures with this verb? For example, hitastigin breytast hratt?

Yes. hitastigin is the definite plural of hitastig (“the temperatures”). Saying Hitastigin breytast hratt means “the temperatures change quickly,” perhaps referring to different readings or in different places. The verb breytast remains in 3rd person plural to match.