It sounds like you are studying Immanuel Kant. In Kant, a hypothetical imperative means "what you should do IF you want to accomplish something else." Example: If you want to make friends, you should be nice to people. In contrast, a categorical imperative means something that you are morally REQUIRED to do. Example: Do not kill innocent people. I do not recall well the different relationships to reason between the hypothetical and categorical imperative, but I think it may be this: hypothetical imperatives are something people can discover/realize by observing the world. For example, you learn that by being nice, you can make friends. Categorical imperatives, in contrast, are something that a person can figure out using "pure reason" -- meaning just using logic, not experience.