1. I can't stay after lessons. I am busy
2. I ca't answer the questions. They are difficult
3. I can't write this word. I don't know it
4. I can't sing the song. I don't know the words
5. I can't clean the blackboard. I have no duster
6. I can't clean the classroom. I have no time (I don't have time)
7.I can't show you my exercise-book. It is at home