Learn Icelandic: Þessi bíll er skítugri en rútan. - This car is dirtier than the bus.

The base adjective is skítugur (dirty). For most adjectives ending in -ugur, we don't add the full -ari to make the comparative. Instead, we just add -ri to the stem, which gives us skítugri.

Does the word en (than) force the noun after it into a specific case?

No, en is a conjunction, not a preposition, so it doesn't trigger a specific case. The second item in the comparison (rútan) just copies the case of the first item (þessi bíll). Since the car is the subject (nominative case), the bus is in the nominative case too.

I sometimes get confused by the word for 'this'. Why is it þessi here instead of þetta?

The word for 'this' changes to match the gender of the noun it points to. Because bíll (car) is a masculine word, you must use the masculine form þessi. You would use the neuter form þetta for neuter nouns, like þetta hús (this house).

Why does rútan end in an -n?