На какую тему письмо? начать можно так
Moscow
Russia
19/12/18
Dear Diana,
Thank you for your letter. I was happy to learn all your news. I got your letter yesterday and decided to reply straightaway.
Основная часть зависит от темы.
That's all me for now. I have to finish writing because I promised my mum I'd clean the flat.
Best wishes,
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