Learn Icelandic: Bíllinn skemmdist þegar ég keyrði heim. - The car got damaged when I drove home.

The verb skemmdist is the middle voice form of skemma (to damage). By adding the -st ending, it takes on a spontaneous meaning, translating perfectly to got damaged. Icelandic frequently uses -st verbs for accidents or sudden changes where the focus is on the event happening, rather than who or what caused it.

How exactly do you build the past tense form skemmdist?

You start with the regular third-person past tense of the active verb. The active verb is skemma, and in the past tense for it, that is skemmdi (it damaged). You simply attach the -st ending to get skemmdist (it got damaged).

I keep mixing up heim and heima. Why is it heim in this sentence?

You use heim because there is active motion towards a destination, which is driving homewards. You only use heima when there is no movement, indicating a stationary location (being at home).

Why is the word for when þegar instead of hvenær?

Þegar is a linking word used to connect two statements, meaning at the time that. You only use hvenær as a question word, like if you were asking a direct question: When did you drive home?