D40 struggle: Am I using it wrong or do I need a new lense?

[tsaraleksi]

The camera's AF gear can tell you when the shot is in focus, but without AF-S it can't do anything to make the lens go into focus.

The problem you're running into is that the human eye is a very versatile seeing device. It adjusts itself (auto exposure, if you will) without you even realizing (the pupil is an aperture) so that two scenes of extraordinarily different amounts of light will look roughly similar to you. Memory and experience are the only ways that you can gauge the amount of light in a room without a light meter.