1. 表示歉意;2. 解释原因;3. 另约时间。
注意：1. 词数120~150;2. 可适当增加细节。
I’m very sorry that I can’t go on a trip to Beijing with you next weekend, which I have promised you. I’m writing to tell you the reason for it.
Just now, my cousin, Li Ming, who went abroad for further education last year, informed me that he would return the next Saturday morning. He asked me to pick him up at the airport with his parents. As you know, we haven’t seen each other for about a year, so I’ll have to help him in the first place.