1.could you 表示比can you更委婉客气的请求,多用于下对上或幼对长,例如：Could you give me some money,mum?
2.may表示允许（might表示更客气的允许）,
May I sit here.
3.should表示劝告或建议,与ought to意思相同.
You should drink more water.