下列各句中均有一处错误，请指出并改正。

1. He asked these students volunteer in the after-school study program.                  ___________

2. My uncle wants to help the poor people in the city by set up a food bank.           ___________

3. Some of my classmates volunteer to sing for the old people to cheer up them.     ___________

4. What do you think could I do for you?                                                               ___________

5. Be a volunteer is great.                                                                                       ___________