Student or Learner
Which question is wrong?
You brought your pencil but no your eraser. How did you forget it?! It's important!
How could you forget it?! It's important!
How could you forgot it?! It's important!
(Not a Teacher)
"How could you forgot it!" is wrong.
"How could you forget it!" or, as bhaisahab said, "How could you have forgotten it!" are the best ways to say this.