Informal vs. Formal Letter Writing

Are "See you soon!"

"Talk to you later!"

"Can't wait to hear back from you!"

informal or formal?

Is

"I look forward to hearing from you."

formal or informal?

Which expressions are used for opening or starting messges, emails or letters?

Which expressions are used for closing or ending letters, messages or emails?

What's the best way to close this letter.

Dear John,

I am writing to thank you for you help last weekend. I really appreciate everything you have done for my family and I. I hope I will be able to pay you back some day in the future.

_____________,

Brian

Which one of these is formal?

How often do you write a message, email or letter to someone?

Who was the last person you wrote a message, email or letter to?