Use the phrase "How can you ___?" when you want to criticize what someone is doing. Asking this shows that you don't think the listener should do this. For example, if someone in your family is listening to loud and annoying music, you can ask:
How can you listen to this?
Because "How can you ___" sounds negative, you should use a different phrase when you actually want to know how a person does something. You ask:
How are you able to do so many things at the same time?