I learned English in school, but I forgot most of it.
So, this is a question that I saw and the correct answer is "I learned English in school, but I have forgotten most of it." However, I do not understand why the former sentence is grammatically incorrect. I tried searching for it, but I could not find it or understand. Someone has asked this as a fill in the blanks on this site but I am looking for the reason and not a yes/no type answer.