a) Class had already begun by the time I got there, so I  quietly took a seat in the back.
b) My group mate was discussing something with the professor when I walked into the room.
c) It was midnight. I had been studying for five straight hours. No wonder I got tired.
d) Millions of years ago, dinosaurs were roaming the earth, but they had become extinct by the time humankind first appeared.
e) I called Kate at nine last night, but she was not at home. She was studying at the library.
f) Kevin suddenly realized that the teacher had asked him a question. He couldn’t answer because he had been daydreaming for the last ten minutes.
g) I had never seen any of Picasso’s paintings before I visited the art museum.
h) The anthropologists left the village when they had collected enough data.
i) While Roger was writing an essay his roommate was cleaning the room, so Roger could not concentrate and got angry.
j) As I had passed the hardest exam and (had) got an excellent mark I felt a know-it-all.