Learn Icelandic: Mannfjöldinn er átta hundruð þúsund íbúar. - The population is eight hundred thousand inhabitants.

In Icelandic, the word for "hundred" (hundrað) acts as a neuter noun. Because we are talking about eight hundreds, it must take the plural form. The plural of hundrað is hundruð.

Why doesn't þúsund also change to a plural form?

Þúsund (thousand) is also a neuter noun, but its plural form is identical to its singular form when used in exact numbers like this. So, one thousand is eitt þúsund, and eight hundred thousand is still átta hundruð þúsund.

How is the word mannfjöldinn put together?

It is a compound of mann- (the combining form of maður, meaning "person" or "man") and fjöldi (meaning "number" or "multitude"). The -nn at the very end is the attached definite article ("the"), giving us "the population".

What is the singular form of íbúar (inhabitants)?

The singular is íbúi (an inhabitant). It is a masculine noun ending in -i. In Icelandic, these types of nouns usually form their plural by replacing the -i with -ar.