Look at this sentence from a book: A tennis player has to / doesn't have to hit the ball inside the white lines. So, if you know how tennis is played, you'll probably say that doesn't have to would be the correct option. Nevertheless, the key from the teacher's book says has to is the correct one... This is the context within which I have to solve the exercise: Modal verbs for rules: Not necessary (but allowed): don't have to Necessary and an obligation: have to the white lines = the lines down the side of a sports area, defining when the ball or the player is in or out As far as I know, in tennis you can hit the ball inside or outside the area... What matters is that the ball hit the ground inside the white lines in your opponent's side of the court... So, have I been playing tennis the wrong way all my life or is the book just forgery?