Car moves and rolls while in drive.But the gas doesnt work. The pedal is loose and when u mash down there is no result

This sound like it might be a transmission issue. If the revs are dropping as you are at idle something is 'Dragging' the revs down and the delay is the engine and transmission trying to catch up with each other. A test you could try is the next time the revs start to die drop it into Park and 'Blip' the gas pedal to get the revs up then put it into Drive to pull away.
You may need to take it to a Transmission Specialist to get them to look at it.

This is when you need to let the clutch out just enough to stop the backwards roll when you take your foot off the brake, then you can move to the gas pedal and when pressing the gas pedal you can release the clutch all the way to get moving. IT is a matter of practice and timing..

Michael, have engine and transmission mounts checked.
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The question is a little vague but I think I know what your referring to. All modern automatic transmissions have torque converters. This is the device that connects the engine to the transmission. All vehicle manufacturers like the trans to be nice and smooth shifting. That's great but it does come with some minor drawbacks. If you are facing slightly down hill and you put your car in reverse but do not step on the gas slightly I can guarantee that you will indeed roll forward. You can eliminate that issue by holding your left foot on the brake while you shift into reverse. Then, as soon as you feel the trans engage, gently depress the gas pedal with your right foot and it will eliminate that forward roll problem.

sensor problem have it scan for any trouble codes

Possibly a fuel problem, check fuel pressure, run some injector cleaner through the tank, check for trouble codes.

sounds LIke Drive Shaft is loose and Maybe bracket is Worn Out

U have tranny issues - better have that checked out before you walk.

Assuming this is a standard transmission, you always have it in 1st gear. Roll back is controlled by balancing the clutch with the gas pedal.

If an automatic transmission, then the vehicle should hold itself on the incline, unless it is a steep incline, to which you may use the gas pedal to hold the vehicle on the slope, and not the brake. And you would always have the transmission in the main drive gear position.

check fluid level by raising car,run untill operating temp,unscrew 5 mm allien key plug ,fluid should be rolling out of hole in not add.
once fluid level is good turn key on,(not running) mash gas pedal tp floor for 15 secs turn key off and on and off again.that resets trans memory