Was ist der Unterschied zwischen 'my friend' und 'a friend' und 'a friend of mine' ?Du kannst auch einfach Beispielsätze geben.

"My friend" is used when talking about a specific friend that has likely been mentioned before. You would also call the friend "my friend".
Example 1: I saw my friend the other day.
Example 2: I want to see you because you're my friend.

"A friend" is used when you're not being specific about who it is or you don't have a certain friend in mind. You may also hear it when someone is saying that another is just a friend, rather than a romantic interest.
Example 1: I saw a friend today.
Example 2: Tom is nice, but I don't want to date him. He's just a friend.
"A friend of mine" is a more formal version of "my friend" and is often used when the friend has not mentioned previously to the other person in a conversation or during introductions.
Example 1: a friend of mine said that this is a good place to eat.
Example 2: this is Steve, a friend of mine.