Hello there,
When I write a letter to someone at beginning of paragraph.
Should I use A or B, which expression is the formal sentence?
A I am writing you about your letter
B I am writing to you about your letter
Thank you,
In British English, 'to' would never be omitted in that sentence. Only sentence B is correct in BrE.
In American English, both are correct. Whether one is more formal I don't know, although I'd guess the omission of 'to' would make it less formal.