How about the distinction between "shall" and "will"?

I'm not 100% sure I understand the traditional usage myself. Suffice it to say that The King's English, by Henry Watson Fowler and Francis George Fowler, devotes about 10 pages to the subject (133 to 154).

"Shall" has almost disappeared from American usage except in questions:

"Shall I get you some coffee?" = "Do you want me to get you some coffee?"

We would almost never hear or say "I shall get you some coffee."