Learn Icelandic: Umferðin er mikil vegna slyssins. - The traffic is heavy because of the accident.

The preposition vegna (because of) requires the noun following it to be in the genitive case. To build slyssins, you take the neuter noun slys (accident), add an 's' for the genitive case to make slyss, and then attach the genitive definite article -ins ("the").

The English translation says "heavy", but doesn't mikil mean "much" or "large"?

Yes! In Icelandic, you do not usually describe traffic as literally "heavy" using the word þungur. Instead, you say there is "much" or "great" traffic, which is why mikil is used here.

Why is the adjective in the form mikil rather than mikill or mikið?

Adjectives must match the noun they describe in gender, number, and case. Umferð (traffic) is a feminine noun in the nominative case, so the adjective takes the feminine nominative singular form, which is mikil.

How does the word umferðin get its meaning of "the traffic"?

In Icelandic, the definite article ("the") attaches directly to the end of the noun instead of being a separate word. The base word is umferð, and adding the feminine singular article -in makes it umferðin.