Hello, I'll also include Darryl's comments. :)
Dear Mary,
I know that you are interested in travelling, so I AM WRITING to tell you about a fascinating film that I watched yesterday. It is a film about Beograd. This city is the capital of Serbia. In my opinion, Beograd, as well as Serbia in general, has a wonderful and picturesque SCENERY OF nature. The city is located at the confluence of the Sava and Danube rivers. Moreover, Beograd has a long and rich history that includes a lot of tragic moments. For example, the city was destroyed about 40 times during the wars and attacks. In spite of all these tragic incidents, there are friendly and hospitable people in Serbia. Furthermore, I believe that each of them loveS the country with all THEIR heart. To sum up, after watching this film I fell in love with the country and decided to spend my summer holiday in Serbia. I will be glad if you join me. IF YOU COULD, PLEASE give me an answer as soon as possible.
Best wishes,
Stasya
***Your writing is very good!! :) I just made small corrections and added a few parts towards the end to make it sound more polite and not too much like a demand. Great job! ;)