if you are trying to move all the verts in a selection to a plane in maya you can either scale the selection along one axis until they are all lined up

or

make sure that "keep component spacing" in the move tool properties is unchecked and that the move tool is in "world mode" and then constrain the movement of all the verts to one axis and snap all the points to a grid or vert by pressing and holding the x or the v key and middle mouse gesturing over the target grid line or vertex.