You are experiencing a problem that I experienced 6 months ago , when I was a beginner
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. Think were you would want to have the hinges , and then place the origin brush ( a cube textured in "ORIGIN" ) . Select both the door and the origin brush and tie to enity (func_door_rotating)
1 image = 1000 words , so :
Hope I helped you ,
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Don't worry , post as many questions as you can , the people here will help you . I was like you when I was a beginner
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[EDIT] Sorry for "know" instead of "now " in the image .