Here are the completed sentences with "should" or "shouldn't":

11. You **shouldn't** stay up too late because it's not good for your health.
12. No one **should** be late for school tomorrow because we will have an exam.
13. I think there **shouldn't** be a garbage dump in front of the restaurant.
14. You **should** go for a check-up regularly.
15. I think you **should** be more tactful.
16. If you have any questions, please write me an email.
17. What **should** I do to learn better?
18. My father thinks that I **shouldn't** be so into computer games.
19. The candidates **should** be here before 10 a.m., or they will be disqualified.
20. You **should** learn about some cultural features of the country you are going to visit.
21. Mary **shouldn't** take ant pills before seeing the doctor.
22. James should apply for the job at this company? - Yes, he should.
23. How much time **should** I spend on this task?