He___to meet us at the station, but didn't see us.
__________ in thought, he almost ran into the car in front of him.
__________ in 1636, Harvard is one of the most famous universities in the United States.
When __________, the museum will be open to the public next year.
The research is so designed that once __________ nothing can be done to change it.
__________ more attention, the trees could have grown better.