Re: My friend John / My friend, John, went with me to the store.
I rarely say this, but I disagree with Bhaisahab. I would not assume that John was your only friend with sentence 1. We regularly use "my friend + name" without pauses or commas, especially when speaking. I can certainly imagine saying to someone:
Originally Posted by bhaisahab
"My friend Sarah is an accountant. My friend Bob is a teacher. My friend Dave is a thriller writer. His friend Bill makes horror films. So some of my friends have really interesting jobs!"
For me "My friend, John, went with me to the store" is simply a sentence which gives slightly more information than "My friend went with me to the store".
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.