Here are the rewritten sentences based on the prompts:

11. You can't make a cake without flour.
=> Unless you **have** flour...

12. We haven’t got any money left, so we can’t have dinner.
=> If we **had** some money, we could have dinner...

13. You should give up drinking to protect your health.
=> If you **would** give up drinking, it would improve your health.

14. Maybe I could go to Singapore to visit you.
=> If I **went** to Singapore, I would visit you...

15. I don’t speak English well enough to work for a foreign company.
=> If I **spoke better English**, I could work for a foreign company.

16. Speak to her only if she speaks to you first.
=> Don’t **speak to her** unless she speaks to you first...

17. Please give the book back to me some time – I’d be grateful.
=> If you **could give** the book back to me, I’d be grateful.

18. I’m living in Italy because I got married to an Italian.
=> If I **hadn't married** an Italian, I wouldn't be living in Italy...

19. We’ve got a broken window because you and your friends were playing football in the back yard!
=> If you **hadn't played** football in the back yard, we wouldn’t have a broken window...

20. I missed the end of the film, so I don’t know who the murderer was.
=> If I **hadn't missed** the end of the film, I would know who the murderer was...