Can you tell me how the female reproductive system work when infertile?

Varies tremendously. Infertility has many causes and thus the reproductive system has many changes when infertile. You are 40 and decreasing egg number and quality are important causes of infertility. We need to make sure the woman is ovulating and releasing an egg. We also need to check for female anatomy and sperm number and function. You should seek the advice of a specialist if you have concerns.