Kant's Grounding for the Metaphysics of Morals

Kant’s view of morality is guided by his belief that moral truth can only be known through the exercise of our own reason. One of the ways we can ascertain how to act morally in any given situation is to apply Kant’s idea of the categorical imperative. The categorical imperative is the rule whereby only those principles that can become universal law should guide your actions (p. 14-15). For example, a deranged murderer comes to your house and asks you where he might find your neighbor, whom he intends to kill upon sight. Kant would say that, despite the damage that might befall your neighbor if you tell the deranged murdered where he is, you should nevertheless tell the murderer the truth. To lie to the murderer would be immoral, because lying can be taken to be a maxim that nobody would want to apply universally. For Kant, whatever our desires, inclinations, and goals might be do not make any difference in our moral decision-making. Though acting according to the categorical imperative is always the goal for Kant, he realizes that we also have one foot in the “world of sense,” in which we follow our own nature and desires. (54-55)
Because the categorical imperative is not applicable to every action we undertake in our daily lives, Kant sets forth the idea of the hypothetical imperative. Hypothetical imperatives are actions that one takes out of practical necessity as a means to attain a certain goal. For example, if you want to get a good job someday, you should go to a good university like GW. (Or, one which is more applicable to my household at the moment: if you want to get rid of the flies in your kitchen, you should probably clean out your rancid trashcan). In Kant’s view of the hypothetical imperative, we act on our desires and inclinations in order to achieve a certain goal that we find beneficial to ourselves.
In other words, the categorical imperative tells us to do x no matter our present circumstances, whereas the hypothetical imperative tells us we can do x, y, or z depending on what situation we might find ourselves in at the moment.