My mom has not driven in 8 years due to poor eyesight. Tonight she decided she wanted to go shopping and started looking for the keys. How do I tell her she can’t drive?

If you feel she should not be driving then you need to do as suggested above if she gets angery so be it she will get over it just make sure someone can take her weekly to do her errands in a short time she will realize it is better.

Does she even have a license any more? If not, just tell her. My mother still did and it took a while to get it taken away. It took 5 accidents and two doctor's statements before the Bureau of Motor Vehicles order her to take a new written and driving test, which she failed. In the meantime, I hid my mother's keys. She found them and tried to bite me when I tried to take them from her. I called my brother and he came over and disconnected the battery cable and distributer cap. As soon as she received her official confirmation that her license was being revoked, due to failure to pass the tests, since I had Power of Attorney over financial matters, I told my brother to sell the car for me. As soon as the deed was done, I just told her that I had sold her car. She was not happy, but that was the end of the matter.

Tell her the cars broke, she probably won't remember anyway.
My mother-in-law is lucky to remember something more than 5 minutes, that's what I would tell her then she'd move onto another subject.