Proper way to use "my" and "mine"

Then I thought why doesn't it say "my colleague"? And it reminds me to think of the difference between these.

This is just an example and I think it is not only the case my/of mine. I also want to know the proper way to use "of" which especially means "someone's". I can't think of anything other examples, but "of" sometime misleads me when I make a sentence.

Can anyone explain this please?

I think I should post this on Discussion, but I don't have confidence in my sentences. So if you don't mind please correct my sentences and explain this question. Thanks.

: The reason the sentence says "a former colleague of mine", is because it is past-tense (the word "former" indicates this). "My colleague" would be used when "John", is your current colleague and sentence would probably place John after it( My colleague John). Of is a very difficult word to explain in my opinion, its placed in a lot of sentences that don't really make sense. For example, "Of course" it sounds correct but "of" should not even be there. "Of" has most use in old writing and poems, in which the grammar was different. For example, "An apple of green", or "Louis of France". Lastly, just a small detail, when saying "Of". It is pronounced

"U-V" not "O-F" because that would sound like the word "off". That's probably the best I can do for an explanation.