Learn Icelandic: Þetta eru fimm nemendur.

In Icelandic, only the numbers 1, 2, 3, and 4 change their form to match the gender and case of the noun. From fimm (five) onwards, the numbers stay exactly the same no matter what they are counting.

Why do we use Þetta (this) with a plural verb eru (are)? Shouldn't it be 'these'?

When you point things out or introduce them in Icelandic, you use the fixed neuter singular pronoun þetta (this), even for multiple items. The verb, however, matches the plural noun, so you get þetta eru (literally 'this are'), which translates as 'these are'.

The plural word nemendur (students) looks a bit unusual. What is the singular form?

The singular form is nemandi (student). Nouns ending in -andi usually describe a person doing an action, and they take a special plural ending that changes the end of the word to -endur.