Learn Icelandic: Hér eru átta gestir núna.

The singular form is gestur, which is a masculine noun. To make it plural, you drop the -ur ending and add the plural ending -ir, giving you gestir.

Why do we use eru instead of er in this sentence?

Eru is the plural form of the verb to be (are). Since the subject is eight guests (átta gestir), which is plural, we must use the plural verb eru rather than the singular er (is).

Does the number átta (eight) change its ending to match the masculine noun gestir?

No, the number átta never changes its form. In Icelandic, the numbers 1 through 4 change to match the gender and case of the noun they describe, but numbers from 5 onwards (like átta) stay exactly the same.

What is the difference between núna and nú?

Both words mean now. Núna specifically emphasizes right now at this exact moment. Nú can also mean now, but it is very often used as a little filler word in sentences, similar to saying well... or so... in English.