5E spell use is very different than 2E (BG2). In AD&D (2E) you chose a certain spells you could cast per day and you had to pick them before the day started. You had WAY more spells memorized, but there were often times the spells would go unused because there wasn't a situation to use them. You may have picked 5 castings of heal or fireball, and a detect thoughts etc. and never had a reason to cast them. Once that spell was used you couldn't cast it again unless you had more selected. In 5E you get spell slots which can be used to cast any spell you have memorized as many times as you want as long as you have a spell slot of the minimum level required to cast the spell.
