Sentence 1: I am a substitute teacher. 
 Sentence 3: I was called to teach for someone whose appendix burst once 
 Sentence 4:  I had to rush to the school to teach for her that day 
 Sentence 5:  The teacher was very thankful that I could help her on short notice 
 Option 1: When I get called to teach, sometimes it is an emergency. 
 Option 2: When I get called to teach, the teachers are never thankful.