If we________ , we can realize the progress we have made in space research.

Einstein  was __ a great scientist. He did a lot for human rights and progress. 

The merchant made me promise not to ______ where the goods were hidden.

He had to pause from time to time to wipe the sweat from his forehead, because the air-conditioning system _________.

The petrol in the car ____ on our way to the beach, so we had to walk there.