My sister has a habit of keeping a diary and writes down ______ happens every day．

— The light is still on in his room. Do you know          ?— Maybe he has to study for a test.

（2014年北京市西城区高三5月二模试题）31. A person who has never tasted bitter won’t know ______ sweet is.

 The mother, who is strict with her two daughters, has a clear list of ________ they are not permitted to do.

 I will never forget the day____ we spent in the old town with small houses.