Jack, under the impression that he was the most popular teacher, was surprised to see all of his students remembered his birthday. (This emphasizes Jack's initial belief and the surprising element of his students remembering.)

It was with great enthusiasm that they transformed the room into a family library. (This conveys the excitement and energy they had while making the transformation.)

I'd prefer not to have burned all the fallen leaves in the garden. (This uses a more polite way to express regret about burning the leaves.)

It is imperative that John put a stop to all the arguments in class. (This emphasizes the importance and urgency of John stopping the arguments.) (Here, "imperative" can be replaced with "essential" or "crucial" for slightly different nuances.)