Does merely imagining masturbation during the daytime in past Ramadans nullify the fast and necessitate making up for it, especially if I did not know at the time that this was what was meant by masturbation?

There is no make-up fast required for the ignorant person who breaks their fast by masturbating, as this matter may be obscure to many who are not well-versed in religious rulings. Masturbation is the expulsion of semen by hand or other means. As for sexual fantasies, it is impermissible to intentionally conjure them or indulge in them; however, the emission of semen solely due to thought and fantasies does not invalidate the fast according to the majority of scholars. Therefore, the questioner's fast is valid.