That sudden switch from cheerful to gloomy...

How can you don't know something you don't know??

/\/\ao said:

How can you don't know something you don't know??

You can.
Call me dumb, but I don't understand which part I don't know about algebra and stuff of maths.

/\/\ao said:

How can you don't know something you don't know??

If you know nothing of a subject, how do you know what specific parts of it you need to work on?

/\/\ao said:

How can you don't know something you don't know??

Plenty, when student can't grasp the basic. Most failure in teaching is due to not understand why the students don't know about something.

/\/\ao said:

How can you don't know something you don't know??

Rummy referred to these as unknown unknowns.