I was taught this at school – one of the few things I remember LOL!
I shall drown and no one will save me.
This means I am going to drown because there’s no one to save me.
I will drown and no one shall save me.
This means I am determined to drown whether anyone tries to save me or not.
So
will is more active – something you make happen, and
shall is passive – it will happen if nothing else happens to prevent it.
Does that make sense? Of course it could be all bollocks!
Dee