Dear...
Thank you very much for your letter inviting to go on a picnic to Cuc Phuong. You know how I want to visit this national park but I'm afraid I can't go with you. In two weeks' time, I'm going tohave a very important examination at the language centre so I will spend my next weekend to review my lessons.
I wish you a nice trip. Give my best regards to your family.
Love,
Lili