The difference is that when you are useing in "my" you have to say what! like "it's my book". and when you're useing in "mine" you don't need to say what and the "mine" will be last in the sentence >>" the book is mine" or "its mine" I hope I helped you (and I also hope that I have no spelling mistake :O)
Your second question isn't syntactic.. (There are many ways to write the same question correctly, so I can't really say a specific way to re-write it..) Second- About your question- Mine is the nominative singular pronoun, used by a speaker in referring to himself or herself. example: This is mine, not yours. My is a form of the possessive case of I used as an modifier adjective. example: My soup is cold Hope i helped you.