Grace
I agree that studying Old Norse throws light on usages in modern English.
I wonder if the expression "I'll larn him" is really incorrect. In Old English 'læran' means 'to teach'. Old Norse is almost the same  - 'læra'. I can't find the Old Norse word for 'learn', but in Old English it is 'leornian'. I believe that 'I'll larn him' really does mean 'I'll teach him', but  this word generally fell out of use to be replaced by 'teach'. When people of later generations, accustomed to the word 'teach', then heard the now antiquated 'larn', they associated it with 'learn' and said it was wrong. Not so. Archaic or anachronistic, yes, but not incorrect. People who say 'larn' are the unconscious inheritors of part of a vocabulary which has been replaced by Norman-French and Latinate terminology. 
I wonder how many other 'incorrect' words and expressions used in English-speaking countries there are. Any suggestions?
Cheers
Jed