Bossy Commands vs Polite Requests
If you’re annoyed with someone who is pestering you, you might utter, “Will you just go away!” In this example you’re being demonstrative. It is clear you want them to leave immediately. You wouldn’t say, “Shall you go away!”
If you want someone to do you a favor, you want to sound polite in your request. “Will you please bring me a glass of wine?” If you said, “Shall you bring me a glass of wine?” They might respond with, “I don’t know, shall I?” and then walk out of the room thinking you’re weird. Then you would have to get your lazy butt off the sofa and get your own wine.
Shall is often used in legal documents to indicate binding obligation. For example, if you read a publishing contract, you will come across this type of sentence:
“The Publisher shall have the sole and exclusive right to publish or to license the Work for publication in the English language, etc.” Even though the word will works fine, shall is the chosen word.
The Future of Future Tense
The verb will is like a crystal ball. It is needed when referring to a future act, state, or condition according to The Chicago Manual of Style. For example: “She will sing the National Anthem before the baseball game.” It’s a done deal. She’s been given the honor and unless she meets with an unfortunate occurrence or gets stage freight, we can expect her to perform. In this case, will is not interchangeable with shall. SO: “She shall sing the Natural Anthem at the baseball game,” indicates that she is capable, or has been given permission, but doesn’t necessarily mean it’s going to happen. Some grammarians believe the future tense is going the way of the buffalo. So, will the future tense disappear? Maybe. Shall the future tense disappear? Its existence is out of our control.
Strunk and White show the difference with very little explanation in their classic grammar book, The Elements of Style. They simply use two examples:
A swimmer going down for the third time cries out, “I shall drown; no one will save me!’
But if that swimmer wanted to drown, he would say, possibly in a quiet, pathetic voice, “I will drown; no one shall save me.”
In closing, I’d like to add, “Will you please share this? I shan’t ask again.”