"he has not been served yet" means that he can still be served, he is still at the same place and someone may come any time to serve this person. "he was not served" refers to a time period which is already closed, so he can't be served in that same time period any more.

"Have you been not" is never correct. "Have you not" is proper English, but very old-fashioned, and is never said in conversation. It is always contracted to "Haven't you", such as "Haven't you eaten breakfast yet?"

Hi! Its simple, when you use "been yet" you are talking about an action that that has not happened but will or might happen in a future. In other words there is the posibility of the action to happen. But, when you use the sentence in past like "was not served" you are talking about an action that has already finish and there is no room for changes. Imagine you hear that in a restaurant..... "He has not been served yet" = He has not recived his dish, but he will" // He was not served = He didn´t recive any service at all. I hope This helps you to understand this a little better =) If you have any further questions feel free to ask me