if it says "italiener" isn't it a plural? And if it is a plural, why is it "du" instead of "i".
No. The demonym for an Italian person is "italiener". The plural is "italienere"
Er du italiener?
Er I italienere?
The definite forms are italieneren (singular) and italienerne (plural).
Remember that the second person plural personal pronoun should always be capitalised: I