can、must may would四个的区别,

The story is about a girl.She must do a lot of homework to do at night,but her mother is very sick in bed.She must first cook for her mother and then go back to do her homework.She may be tired but she must do it everyday.I respect her with her spirit,and I wish I may help her,cause she is really tried everday,but she told me,that's she must do,cause she loves her mother.