Some scientists believe everything we ever learned is stored away in our brain waiting on the synapses to fire in the correct sequence to recover them. Nothing is "forgotten", it is just buried in dusty, unused files down in the basement.
Some scientist believe in global warming, the Earth is flat, and the sun revolves around our planet. Do I need to discuss phrenology, leeching, the certain belief that applying horse dung would heal cuts? Scientist are wrong all of the time.
I studied French in high school and the first years of college. At one point I could speak it on about a fourth grade level. I could read it at about a high school level. When I was on France about 15 years ago I could speak at about a kindergarten level and read at about fourth grade. Now? Forty years removed from my French education I can speak a few phrases, understand none and read only a few words.
If I run across French in ordinary circumstances -- a foreign film or a passage in a book, I have little to no comprehension. It might as well be Hungarian or Cherokee. Strangely, every time I go to a foreign country my French comes back a little. I'm in China and I think of French phrases. In Mexico it was the same. In fact, I think my few phrases and words of Spanish were mumbled with a French accent. Why is that?
It is weird because anyone who has read any of my musings will readily attest I can barely communicate in coherent English.