If we had become aware of danger earlier, we _____ to stay in that temporary shelter.

 It is hard for me to imagine what I would be doing today if I ________ in love, at the age of seven, with the Melinda Cox Library in my hometown. 

Nobody knows how people first came to these islands. They _____ from South America on rafts.

It is written clearly in the requirements that all passengers _______ check onto board thirty minutes before the plane takes off．

— Why didn’t you go to the dance last night? — If I             tired, I would have.