"Retain" means to hold or to keep possession of. “Keep” can be used more often, however it means relatively the same thing as “retain”. However, “retain” can also mean to continue to do something. “Maintain” to keep the same level of something. Examples: Retain: I have studied very hard for the test, and I have found that I am doing well retaining the information. I tried to win, but that was difficult when my team was retaining information from me. Keep: I have to keep trying or else I will fail! I keep my grandmother’s necklace around my neck all the time. Maintain: The human body must maintain its homeostasis and metabolism. If you maintain this behavior, you will not be allowed to return to school.