Taking large amounts of folic acid when B12 is low, masks the symptoms and therefore damage to the nervous system progresses, without intervention. This is a complex biochemical system and putting up all the chemistry is just complicated and not necessary for most people reading here. It can be found on the net easily.
With the food fortification in our country as it is, this issue comes up more now. That is why getting a serum B12 level is important, if someone has neurological symptoms.
And this is why in this country folic acid 1mg or higher requires a doctor's prescription. (but sadly not all doctors screen for B12 levels before giving that prescription out, or even worse, they are allowing low levels as "normal", when in other countries that cut off is much higher