Learn Icelandic: Þvoið báðar hendurnar strax!

The word for 'both' changes depending on the gender and case of the noun it describes. Here, it describes hendurnar (the hands). The singular word for hand (hönd) is a feminine noun. Because the hands are the direct object receiving the action of being washed, they are in the accusative case. The feminine, accusative form of 'both' is báðar.

Why does hendurnar have the definite article attached to the end when the English translation just says 'both hands'?

In Icelandic, whenever you use the word for 'both' (báðir), the noun that follows it must always have the definite article (the 'the' ending) attached to it. So literally, Icelandic says 'both the hands' (báðar hendurnar), even though in English we simply say 'both hands'.

Why does the verb þvo (to wash) end in -ið here? Is it a command?

Yes, þvoið is a command (the imperative form). Specifically, the -ið ending means it is a plural command, used when you are telling two or more people to wash their hands. If you were giving this order to just one person, you would use the singular form: Þvoðu.

If the singular word for hand is hönd, why does it start with hend- in this sentence?