Learn Icelandic: Fjörutíu Íslendingar fljúga frá Akureyri í dag. - Forty Icelanders fly from Akureyri today.

No, you do not! While numbers 1 through 4 change to match gender and case, the 'tens' from 30 to 100 (like fjörutíu) never decline. They stay exactly the same no matter what noun they describe.

Why is forty fjörutíu instead of just combining the standard words for four and ten?

It actually is a combination of four and ten, just using a slightly shifted form. It comes from an old grammatical form of four squished together with ten (tíu). You will notice similar sound shifts in other tens, like þrjátíu (thirty) instead of þrjútíu.

How do we get the word Íslendingar from Iceland?

It is made by adding the suffix -ingur to the country name Ísland, which is a common way to say 'a person from a place'. The singular is Íslendingur (an Icelander), and because it is a regular masculine noun, the plural becomes Íslendingar.

Does the preposition frá change the case of the town Akureyri?

Yes, the preposition frá (from) always requires the noun following it to be in the dative case. However, the town name Akureyri has the exact same form in the nominative, accusative, and dative cases, so it doesn't look any different here!