She was so angry at ____ he was doing ___ she walked out without a word.

I have learned from my bitter experience　　　we don’t knowwe’ve got until it’s gone.

—There is a girl in front of our classroom. Do you know       ? —She is an English teacher.

–-- Can you tell me ____________ in the future? --- Sure.  It must be much better .

The Math problem is too difficult , we really don’t know ________ to do it .