Mages get experience point by killing things, completing quests, and memorizing spells. I played a mage in BG1 but am a monk in BG2. I can't think off hand of any other ways mages acquire experience. When your mage has the necessary number of experience points, he will level. Then you should have more spells available to you based on your intelligence level and any modifiers you have. Did that help?
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O_H_F
