newbie:user control disappear

Hi! I have a main program, which use User control to display data. In fact, I am using the user control cs file from someone else for the learning purpose. I have to manually add all code ( construction, resixe, position..) of the user control to the main program but to my surprise, whenever I change the design mode of the main program, all the code for the user control onject disappear and also it is not in the control anymore. Can anyone tell me the mechanism of user control. Regards.