"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.
आयेगा and जाएगा are the masculine singular third-person forms which you use when talking about someone else other than yourself and the listener. They are also used with तू.
आऊँगा and जाऊँगा are the masculine first-person forms which you use when talking about yourself.
राज आज आयेगा और मैं कल आऊँगा - Raj will come today and I'll come tomorrow.