Can the DSLR NIKON D3100 use Manual Focus Lens?

Looking to buy a new DSLR.

Manual focus? Yes

you can turn autofocus on and off on the lens

The entry level Nikon dSLR's are designed for those users just staring out and who do not already have a camera system with any legacy lenses, thus the less than $1000 cost for camera and lens. They use all the recent AF-S auto-focus lenses. If you do not mind manually focusing lenses, then the older AF lenses can be used.

Older AI manual focus lenses can be used with any of the more advanced cameras like the new D7000 or older D70, D80 and D90 bodies.

If you are asking specifically if you can use AF-S lenses in the manual mode, that answer is yes, you can

Yes, because there's no autofocus motor in the camera body. So the motor is in the lens. With a camera like the D-200, or D-300 the motor is in the camera. I'd take a look at a used D200, or even a D300. A lot more camera in the long run.

Yes you can use manual focus lens on the D3100, but it will not meter with the lens. The camera histogram can be use to determine the exposure though.