Should I write a comma in the next examples?

1) Hello friend,

[text of a letter].
2) Hello, friend,

[the same].
I have noticed that many people don't use a comma. They just say, "Hello someone, ". Is it right?

And one more question. Should I use commas after and before "please"?

1a) Can you, please, help me?
1b) Can you please help me?

2a) Can you help me, please?
2b) Can you help me please?

3a) Please, help me.
3b) Please help me.

Which sentences are correct?

I'm sorry if you have been over this before but I was searching and didn't find the answer.
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Comments  (Page 3) 
Thank you, Archaic. It is easy for me to memorize because we have the same rule in Russian. Emotion: smile I was not quite sure whether it works in English too. Now I am. However, I think this rule works in many languages but, unfortunately, not everyone uses it.