1) My father has just been operated on. <--this is correct. You must have the word "on" at the end. 2) The doctor has just operated on my father. <--correct (but be sure to capitalize the first word in a sentence). 3) The doctor has just operated on his knee. <--this sentence, all by itself and without other context, means that the doctor has operated on the doctor's knee. It's possible I guess, but not likely. If you mean to say that the doctor has operated on your father's knee then you would have to add that to the sentence or provide the information elsewhere. For example..."I took my father to the hospital. The doctor has just operated on his knee."... You can see that I provide context in my example, and it is obvious that the doctor is operating on the father's knee.

Hello Amirhossein! You would nave to use the preposition "on" after "operated" to explain that the surgery was done to the person or to that afflicted area. In this context the word "operated" is a verb that is being done to a noun and so the preposition is needed. I hope that helps!

1. My father has just been operated on (EDIT: another more natural way to say this is "My father just had an operation) 2. This is fine 3. This is fine