It is（或was）…that …句型中若去掉it is （或was）和that不影响句意,则此句式为强调句；
若去掉it is（或was）和that后句意不完整,则此句为主语从句.
It was about 600 years ago thatthe first clock with a face and an hour hand was made.（强调句）
It is possible that he has stolen the car.（主语从句）