General
I am writing regarding / on account of / on the subject of … (formal / semi-formal)
I thought I’d drop you a line to say… (informal)
I am writing in connection with… (formal / semi-formal)
Referring to previous communication
Thank you for your letter requesting… (formal and semi-formal letters)
I am writing in reply to your letter dated 8th June… (formal)
I just got your letter and…(informal)
Complaints
I am writing to complain about … (formal / semi-formal)
I am sorry to have to say this, but… (semi-formal / informal)
I am writing this letter as I wish to express my strong dissatisfaction with… (formal)
Asking for information and making requests
I am writing to inquire about (formal / semi-formal)
I would appreciate it if you could give me some information about… (formal / semi-formal)
I was wondering if you could help me with… (informal)
Apologising
I am writing to apologise for… (formal / semi-formal)
I’m writing to say that I’m really sorry about… (informal)
I am writing to offer a sincere apology for… (formal)