Learn Icelandic: Hvor höndin er vinstri og hvor er hægri? - Which hand is left and which is right?

Because humans only have two hands! In Icelandic, hvor is specifically used when choosing between exactly two options. If you were asking "which" out of three or more things, you would use hver.

Why is the definite article attached to höndin after hvor?

When hvor is placed directly in front of a noun to mean "which of the two", the noun must have the definite article attached to it. That is why we use höndin (the hand) instead of just hönd (a hand).

Why does hvor stay exactly the same in the second half of the sentence (og hvor er hægri)?

The second hvor is acting as a pronoun standing in for the missing word höndin. Because we already know we are talking about a feminine noun (the hand), hvor stays in the feminine singular form to match it.

Why don't the adjectives vinstri (left) and hægri (right) have special feminine endings to match höndin?

Words for left and right (vinstri and hægri) are special adjectives in Icelandic. They do not change their endings to match gender or case like most adjectives do, so they stay exactly the same here.