It wasn't necessary for you to do that exercise.
->You didn't have to do that exercise (jesli nie zrobiles)
->You needn't have done... (jesli nie musiales a zrobiles)
I'm sure Ann spends all her money on clothes.
->Ann is sure to spend all her money on clothes.
Was it necessaty for you to say that?
->Did you have to do that?
I'm sure he knew what he was doing?
->He must have known what he was doing.
Perhaps she's busy at the moment
->She might be busy at the moment.