can: 从客观角度陈述人们都了解的客观事实

　　●can 表示客观能力, 与先天、后天能力无关。

　　Birds can fly up into the sky easily.

　　鸟儿可以轻易地飞到天上去。

　　Luke can speak Chinese very fluently.

　　鲁克的汉语说得很流利。

　　●can表示人人都了解的客观可能性或听话人已知的客观事实。

　　If you are brave enough, all difficulties can be overcome.

　　如果你不畏艰险，所有的困难都一定会克服。

　　Correct ideas can be transformed into a material force that changes the world.

　　正确的理念一定能够转化为改变世界的物质力量。

　　I hear that you are going to Thailand. It can be dangerous to be there, I think.

　　听说你要去泰国，我看到那儿去挺危险的。

　　may: 表示说听双方均不确切，相当于I am not sure

　　●推测或了解可能性

　　The man you are looking for may be in that room.

　　May I have a talk with you?

　　He may never succeed without your help.

　　●基于may自身的不确定性，常置于句首，用来表示祝愿

　　May success attend you! 祝你成功！

　　(May) God bless you! 愿上帝保佑你！

　　●You can/may come at any time. (口语中can和may从不同含义角度均可使用。)

　　ought to: 从客观角度表示安排、责任或义务

　　●You ought to do the cleaning after school BECause you are on duty.

　　你放学后应该做卫生，因为今天你值日。

　　You ought not to write so carelessly.

　　(It is the students’ responsibility.学生应该做好的事)

　　Everybody should /ought to help each other in the study, shouldn’t / oughtn’t they?

　　(在反义疑问句中should和ought to有时可以前后交叉使用，起到主客观互补的作用)