From ert meaning "[you] are" + þú meaning "you".
- you are, are you
- Hver ertu?
- Who are you?
- Ertu ekki þreyttur? Þú hefur ekki sofið neitt.
- Aren't you tired? You haven't slept at all.
- Ertu is much more used than ert þú which is most often used if the speaker wishes to sound very formal, or if he intends to add emphasis to "þú":
- Er þetta þú??
- Is this you??