Basement Framing
#1

When framing the basement, is it necessary to nail or screw the 2x4 against the concrete wall/floor?
#3
It's best to leave a bit of a gap between the studs and the concrete wall -- perhaps an inch. The gap will vary, since your stud wall will be straight, plumb and square, and your concrete wall is none of these.
Use pressure-treated 2x4s against the concrete floor, and adhere them with nails you shoot in. If you're going to build the wall lying on the floor and stand it up, make sure to build the wall a bit shorter than the wall height or you'll never be able to stand it up. Alternatively, you can install the sole and top plates first, and toenail the studs in one at a time to fit.
Have fun. Study hard before you start.
Use pressure-treated 2x4s against the concrete floor, and adhere them with nails you shoot in. If you're going to build the wall lying on the floor and stand it up, make sure to build the wall a bit shorter than the wall height or you'll never be able to stand it up. Alternatively, you can install the sole and top plates first, and toenail the studs in one at a time to fit.
Have fun. Study hard before you start.