This is a question about “sentence order”. Here is the thing.

'The writer's options', a writing guidebook says: there are two basic facts about sentence order. The first is that the sentence has naturally emphatic positions. the end of the sentence is the most emphatic positions; the beginning of the sentence is the next in emphasis; and the middle of the sentence is least emphatic.''

I've been puzzled with this principle. Precisely, I just don't know what the 'being emphatic' exactly means. For example, I just don’t know in what occasion I should say 'tomorrow I will tell you a story.' and in what occasion ,I should say' I will tell you a story tomorrow'?

According to the principle, 'tomorrow' in the end is more emphatic than 'tomorrow' in the beginning. but to me, the two sentence is same in every ways. So what is the difference between them? I think they are same. Am I right? Thanks for your help.
With the simple sentences you have provided, they do indeed sound much alike, but when writing a longer narrative, we generally begin a sentence with old information and save the new information, or the surprise, or the conclusion, for the end of the sentence:

Uncle Bob bought a new story book. The book was expensive. It was so expensive that Bob had no money left for lunch. That is why he robbed the bank and shot the teller.

Or you can look at it this way:

MM: What are we going to do tomorrow, Uncle Bob?
Bob: Tomorrow I will tell you a story.

MM: When will you tell us another story, Uncle Bob?
Bob: I will tell you a story tomorrow.
Thank you! Mister Micawber Emotion: smile