"When will you come, Julia?"
Translation:तुम कब आओगी, जूलिया?
This is a very basic question which I should know at this stage but why are we referring to Julia as तुम not तु ? I think that तुम is plural and तु is singular so it seems that we should be using तु . Maybe तुम is not plural. I can't tell from the verb because the ending is feminine and so it is the same for both singular and plural.
As Anand309040 says, तुम is also a singular form. Hindi has 3 levels of formality- तू, तुम and आप. आप is the plural form but can also be used for referring to a single person when you are being respectful or formal.
As a side-note, though Duolingo teaches the तू form so much, you would do well to steer clear of it in actual conversations. It is grammatically simpler because it has similar conjugations as वह, यह etc but it is seen as disrespectful. It is only to be used among very close friends and family.
तुम and तु are both informal addressing of 'you'. But तु is more intimate people like your own kids, spouse etc. But तुम you can use for younger people who are strangers to you.