17. This sentence indicates that at 3 p.m. yesterday, the movie was in the process of being shown on TV.
18. This sentence means that currently, the process of fixing the E-bike is hap****ng, and the elder brother is the one doing the fixing.
19. This is a question asking if it is possible for Tom to write a song, emphasizing the potential action in the p***ive voice.
20. This sentence indicates that students are required to complete the test, and it highlights the necessity of the action being performed by the students.

17. The movie was being shown on TV at 3 p.m yesterday.
 * They were showing the movie on TV at 3 p.m yesterday.
18. My E-bike is now being fixed by my elder brother.
 * My elder brother is now fixing my E-bike.
19. Could a song be written by Tom?
 * Could Tom write a song?
20. The test has to be done by students on Monday morning.
 * Students have to do the test on Monday morning.