1.used to express the time: at express a short while (or a period or moment someday)[like hour]. for example：at 8 o'clock ， commonly used phrases like：at noon, at night, at midnight, at the end of, at that time, at the beginning of, at the age of, at Christmas, at New Year etc. in express a period of time[like month or year]， for instance：in the morning, in the afternoon, in the evening, in October, in 1998, in summer, in the past, in the future etc. on often related to the date[like someday], for example：on Monday, on Christmas morning, on the following, on May Day, on a warm morning etc. 2.used to express the Geographic Position . at- express a little place or a piont, example:I met him at the post-office. A little place on the Pacific Ocean. in- express within limits or a lot of room, example:Changchun is in the northeast of China. He lives in Shanghai. on- express to touch at one end or side; lie adjacent. or a place near by， example: Mongolia is on the north of China. New York is on the Hudson River. to- express outside the pale of surroundings. example：Japan is to the east of China.