英语考试复习提纲中的改错题,有几道不太会做,求教下懂英语的,

英语考试复习提纲中的改错题,有几道不太会做,求教下懂英语的,
一下6道句子,每句中有一处是错误的并改正.
1.You've only just had dinner.You may not be hungry so soon.
2.He needs do it at once.
3.Xiao Li ought to write an essay yesterday,so he couldn't come to the meeting.
4.You mustn't read the paper if you don't want to.
5.He dare not to go up the mountain alone.
6.I haven't got my keys.Oh,I must leave them in the office.