I'm not too sure about the exact grammar rules since I never learned specific English grammar per se in school, but I can give you some examples: "My dog is big and fluffy." "That car in the driveway is mine."

That is my hat.= That hat is mine. A: My computer isn't working.(here you can NOT substitute "mine" for "my" B: Here, you can use mine ( that is the more likely, but it is the equivalent of "you can use my computer")

'My' and 'mine' are different types of pronouns. We have two sets of possessive pronouns: 1. My, your, his, her, its, our, their 2. Mine, yours, his, hers, its, ours, theirs As you can see, these two sets of pronouns are fairly similar - in most cases, the pronouns in the second set are the same as in the first, with the addition of the 's' at the end. The only pronoun which is noticeably different is the one for 'I' - it is 'my' in the first set, and 'mine' in the second. So, what's the difference? It's quite simple - the word 'my' (like her, our and their) can't stand alone as the subject or object of a sentence. It has to be followed by a noun, for example 'my house'. The word 'mine' (like hers, ours and theirs) can stand alone as a subject or object, for example, 'It's mine'. Here are some more examples: "This isn't my coat. That one over there is mine." "My book isn't on the table. Mine is on on the shelf." I hope that helps.

In general, 'my' is used with the noun it is describing and comes before that noun. 'Mine' is used after the noun in the same sentence or may replace the noun if it has been already mentioned in a previous sentence.