Learn Icelandic: Tvær stelpur eru úti.

In Icelandic, the numbers 1, 2, 3, and 4 change their form to match the gender of the noun they count. Because stelpa (girl) is a feminine noun, we have to use the feminine form of "two", which is tvær. (Tveir is the masculine form).

How does the word stelpa change to become plural?

Stelpa ends in -a, which is a very common ending for feminine words. To make these words plural, you drop the -a and add -ur. That gives us stelpur (girls).

Why do we use the verb eru in this sentence?

Eru is the plural form of the verb vera (to be) used for "they", "we", and "you (plural)". Because "two girls" acts as a "they" subject, we use eru to mean "are".

I've also seen the word út for "outside". Why is it úti here?

It depends on whether there is movement. You use úti when someone is simply located outside, like the girls in this sentence. You use út when there is motion towards the outside, like walking out a door.