ok i have an automatic, and when i back up and then put my car on drive, and step on the throttle the car rev's up but it doesnt move at all, and its on drive. i dont know what the problem is, and there is another problem, i just push the boot down from n to d without holding the button on the side of the boot to change it. i know its not suppose to do that but i dont really know what the problem is.

 

In the transmission there are little plastic(cheap) stoppers that determine what gear you are in. You probably broke one of the plastic pieces. I had the same problem. You have to just replace the plastic pieces

 

all maxima autos let you move from N to D without pressing the button so dont worry about that

as for putting it in D and it just revs, search for "shifter bushing" many people have this problem on the automatics and it is probably what you're describing