Why does "Tom's friend" convey a possessive relationship,
while "Tom's a friend" serves as an introduction?
When we figure out that 's' is about possession, and when is 's' a letter 'is'?The apostrophe followed by the letter 's' is possessive. So 'Tom's friend' is a person—a friend of Tom.
This is a contracted form of Tom is a friend (of mine). It doesn't work especially well as an introduction.
Note the last part I added to your question. The basic explanation is:WhenHow can we figure out that 's' is about possession, andwhen is 's' a letternot the verb 'is' or even the verb "has"?