In Kant's ethics, what is his conception of moral worth and how does it relate to duty?

Kant repeatedly emphasises the question of moral worth. He declares that it is not mere actions (‘which one sees’) that are of significance when we contemplate what it is that contains moral worth, but rather the inner principles (‘that one does not see’) that contain moral worth. The point that Kant is making here is that moral worth does not consist in the immediacy of actions as such: one can make a donation to charity, but does that ascribe moral worth to the action? On Kant’s view, the answer is no. What is significant are the principles the person has acted upon in donating money to the charity organisation. And since duty is the only thing that contains moral worth, the agent must be motivated by duty alone.