Sure! Here are the sentences completed with the correct verb forms:

12. The manager **will be** very angry if I **am** late for work.

13. You **will have** to work very hard if you **take** this course.

14. I **will go** to see the doctor if I **do not feel** well tomorrow.

15. They can't hear you unless you **speak** louder.

16. If they **win** this match, they **will be** the champions.

17. We **will have** plenty of time if we **arrive** there early.

18. What **will Jane say** if Jim **tells** her the truth?

19. If Peter **forgets** to write his essay, the teacher **will give** him a low mark.

20. The boys **will shiver** with cold if they **swim** in this lake.

21. The door **will not lock** unless you **press** the green button.

22. Jim won't be late if he **takes** the bus at 6 o'clock.

23. Mary isn't home, but if you **want** to leave her a message, I **will give** it to her.

24. If Jack **cleans** the floor, I **will do** the washing.

25. The children **will be** happy if you **give** them some sweets.

12. The manager will be very angry if I am late for work.

13. You will have to work very hard if you take this course.

14. I will go to see the doctor if I do not feel well tomorrow.

15. They can't hear you unless you speak louder.