题目内容：

I am going to visit my grandparents on the morning of sunday.时间介词为什么不用in 而用on.

优质解答

morning,noon,afternoon,evening,night这些词只要前头有形容词,或者有后置定语（即只要有定语修饰）那么前面的介词一律用介词on,无论是Saturday或是early.例如：on Monday morningon a cold nighton the morning of ...