My lens won’t focus!
Older Nikon lenses may support autofocus, but not have AF motors built in; the focus is driven by a motor in the body. But Nikon D3000-series and D5000-series bodies don’t have AF motors, so they can’t drive these lenses. Newer AF-S lenses have AF motors built in so work fine.
I’m sure my focus is out
If the lens or AF sensor is misaligned, so that even though the AF point lights up in the viewfinder, the object you focused on isn’t quite sharp, the AF Fine-Tune function on Nikon’s enthusiast and pro bodies can correct this. You can also set up a simple test rig with a steel rule (see issue 13).