1. Mr. Anderson: All, Mr. Smith is sick today, and I will be his substitute teacher.2. Your past teacher, Mr. Lin, will be gone for the rest of the year, and I will be substituting for him.3. I am just a substitute teacher for a day while Mr. James is away.
a substitute teacher我幫他代課 = I will be substituting for him. = I will be his substitute teacher.我只是代課老師 = I am just a substitute teacher.