Dear XX:
I'm here to apply for your advertisement for a substitute Chinese teacher online. Before, whether it was last year1February or this May, I had applied for the vacant position of Chinese teacher online. However, for some reason, the advertisement was cancelled. This time, I applied for the position of Chinese substitute teacher again. If you really need one, I look forward to the interview.
You are sincere,
XXX
The second letter:
Dear XX:
I am very interested in your advertisement for a Chinese teacher published on the Internet on May 23rd. I regret that I lost the opportunity to be a substitute Chinese teacher in your school at the beginning of this year, because I was busy with other work at that time. Now I see your's advertisement again. I hope I can seize this opportunity and do my best for this job. I'm looking forward to the interview.
You are sincere,
XXX