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I’m writing to (thank/ tell/ ask/ congratulate/ apologize/ etc.)

I’m writing to thank you very much for the nice post card…

Many thanks for your letter…

I was very glad to get your letter…

Thanks for your recent letter. It was good to hear from you

I’ve been meaning to write to you for ages but somehow just haven’t been able to find the time I really should have written sooner…

time I really should have written sooner…

I’m sorry I haven’t written for so long but I’ve been really busy with…- You are asking me about… I'll do my best to answer your questions.  That’s about all I can tell you on this problem.