in the morning =早上= 是一般用法
e.g. In the morning, I have my breakfast.
on the morning =那天早上=特別日子才用, on the morning 後寫出是那一天
e.g. On the morning of my birthday, I got up early and had my breakfast.
In the morning 在上午(一般用法)Woke up in the morning:在早上起床Not having breakfast in the morning:上午沒有吃早餐
On the morning 較少見,似是未完成的句子,應帶有想表達on the morning XXX之意思如下:Breakfast on the morning tram:早班電車上的早餐I am going home on the morning train:我乘坐著早班火車回家