You added a new Facebook friend and the two of you have been posting a lot of messages to each other. Your wife asks who he is. He's someone who you knew in elementary school. You say this.
He's an old childhood friend of mine.
A "childhood friend" is someone who you were friends with when you both were young, like 10 years old or younger. It's usually someone who lived nearby, whose parents were friends with your parents, or whose house you stayed over at a lot.
It sometimes sounds funny to say "my friend" when you're telling people about someone who's your friend. That's because it can make it sound like you only have one friend. So we also use the phrase "a friend of mine":
I'm meeting a friend of mine for dinner at around eight.
Here's a general guideline for whether to use "my friend" or "a friend of mine":
- If the person you're speaking to already knows which friend you're talking about, use "my friend".
- If the person you're speaking to hasn't met your friend, or probably doesn't know which friend you mean, use "a friend of mine".
And to help you remember, here's an example that uses both — first "a friend of mine" and then "my friend":
I went out to a bar with a friend of mine from college. We were talking at the bar and my friend asks me, "Hey, what's that black spot on your forehead?"
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