When will you use shall or will in a sentence?

Answer 1

"Shall" is used with "I" or "we" to tell about the future. Similarly, "will" is used with "you","he","she","it" or "they" for the prediction.

whereas "shall" is used with "you","he","she","it" or "they" for threatening or forcefully saying something. "will" is used with "I" or "we" for the similar purpose.

For example - "it will break" indicates the prediction while "it shall break" indicates the threat or guarantee of the action.

Answer 2

Must is something that you inevitably have to do, like for example:

"You must finish your homework."

Shall, is something you will do, but it's almost the same as must, in an also orderly tone, for example:

"You shall finish your homework."

However, the thing is, must has a more important feeling to it. Shall does have a 100% have-to-do feeling, but must is even more so. In other words, when it's something very important, and have definitely do, then you use must. However, if it is not so important, but you still have to do it, you can use shall.

This is just in my opinion, and it might not be right. For a more detailed answer, I suggest you go to wikipedia, or ask some other people.