Here are the corrections for each sentence with mistakes:

1. **He said that he will be there again the day after.**
Correction: "would" instead of "will"
Revised: He said that he **would** be there again the day after.

2. **My mother asked me how many classes were there in my school.**
Correction: "there" should be "are"
Revised: My mother asked me how many classes **are** in my school.

3. **Betty said Bill that she didn’t like that film.**
Correction: "said" should be "said to"
Revised: Betty said **to** Bill that she didn’t like that film.

4. **When Santa was sick with the flu, his mother made him to eat chicken soup and rest in bed.**
Correction: Remove "to"
Revised: When Santa was sick with the flu, his mother made him eat chicken soup and rest in bed.

5. **Mary has called her teacher yesterday to tell him about her problems.**
Correction: "has called" should be "called"
Revised: Mary **called** her teacher yesterday to tell him about her problems.

6. **Jennifer said that they are having champagne that night.**
Correction: "are" should be "were"
Revised: Jennifer said that they **were** having champagne that night.

7. **The Prime Minister said that there were so much difficulties ahead.**
Correction: "were" should be "was"
Revised: The Prime Minister said that there **was** so much difficulty ahead.

8. **Mother said to Tom she had something show him.**
Correction: "show" should be "to show"
Revised: Mother said to Tom she had something **to show** him.

9. **He asked me how long does it take to get to school every day.**
Correction: "does" should be "did"
Revised: He asked me how long **did** it take to get to school every day.

10. **The water was not enough warm for the children to swim in.**
Correction: "enough" should be "warm enough"
Revised: The water was not **warm enough** for the children to swim in.

11. **The taxi driver told the man to don’t allow his disobedient to hang out the window.**
Correction: "don’t" should be "not" and "disobedient" should be "disobedient son"
Revised: The taxi driver told the man **not** to allow his disobedient son to hang out the window.

12. **I asked to him whose car he had borrowed the night before.**
Correction: "to him" should be "him"
Revised: I asked **him** whose car he had borrowed the night before.

13. **They said that they went to the supermarket before coming home.**
Correction: "went" should be "had gone"
Revised: They said that they **had gone** to the supermarket before coming home.

14. **The policemen asked me what did happen last night but I was unable to tell them.**
Correction: "did happen" should be "happened"
Revised: The policemen asked me what **happened** last night but I was unable to tell them.

15. **After driving for twenty miles, he suddenly realized that he has been driving in the wrong direction.**
Correction: "has" should be "had"
Revised: After driving for twenty miles, he suddenly realized that he **had** been driving in the wrong direction.

16. **Janet said she would borrow me her new camera if I wanted to use it on my trip to Europe.**
Correction: "borrow me" should be "lend me"
Revised: Janet said she would **lend me** her new camera if I wanted to use it on my trip to Europe.

17. **After she had bought sherself a new automobile, she sold her bicycle.**
Correction: "sherself" should be "herself"
Revised: After she had bought **herself** a new automobile, she sold her bicycle.

18. **George has not completed the assignment yet and Mary hasn’t neither.**
Correction: "neither" should be "either"
Revised: George has not completed the assignment yet and Mary hasn’t **either**.

19. **After John eaten dinner, he wrote several letters and went to bed.**
Correction: "eaten" should be "had eaten"
Revised: After John **had eaten** dinner, he wrote several letters and went to bed.

20. **Jimmy said that if he had to do another homework tonight, he would not be able to attend the concert.**
Correction: "another homework" should be "another piece of homework"
Revised: Jimmy said that if he had to do **another piece of homework** tonight, he would not be able to attend the concert.

Total corrected: 20/20.