Why doesn't this lens work with my nikon D5000?

I have a AF 50mm f/1.8
but it won't autofocus… What's up with that?

Added (1). It was a gift… I can't really return it. I'm fine with manually focusing it's just a bit of a process.

What is up with that, is you did not do your homework.

The AF-S 50 mm f/1.8 will auto-focus on your D5000. The older AF 50 mm f/1.8 will not. You will have to manually focus with any of the older AF cameras. From now on, buy AF-S lenses

Your camera does not have a focusing motor in the body. Therefore it will only autofocus with AF-S lenses.

Only the more expensive Nikon AF-S lenses will auto focus on your D5000.

Perhaps you could exchange it for the new Nikon AF-S 50mm f1.8 lens and pay the difference.