There are plenty of synonyms for the phrase "study at command", which refers to the ability to study or learn something quickly and easily. Some synonyms for this phrase include "quick study", "apt learner", "fast learner", "sharp mind", "eager learner", "keen mind", "intelligent student", "bright mind", and "clever student". These terms all describe someone who has a natural ability or aptitude for learning and can quickly pick up new concepts and skills. Whether you're trying to describe yourself or someone else, using these synonyms can help to emphasize the individual's intelligence and capability when it comes to studying and learning.