I am having trouble with articles. Here is the context:
You are now changing your username. A/the new username must be different.
I think it would be just fine to just use your there, but I want to know which article is okay to be used there.
As it is not definite—not set yet—I think I should use a there, but at the same time as it is kind of The new username (you will have happened to use) must be different, I think I should use the there.