in the morning, in the afternoon 如果在这两个短语中加入任何修饰词其前面的介词都要改为on, 如:on a cold morning, on the morning of July 14th [误] We visited the old man in Sunday afternoon. [正] We visited the old man on Sunday afternoon. [析] in the morning, in the afternoon 如果在这两个短语中加入任何修饰词其前面的介词都要改为on, 如:on a cold morning, on the morning of July 14th