a) I did not have to buy sugar. b) You need not have bought sugar.
a) We did not have to worry about her. b) You need not have worried about us.
a) You need not have gone there. b) We did not have to go there.
a) I did not have to go to the library. b) You need not have gone to the library.
a) You need not have gone downtown. b) I did not have to go downtown.
a) You need not have checked for spelling mistakes. b) I did not have to check for spelling mistakes.
a) I did not have to ring him up. b) I need not have rung him up.