Simple question about moving

I struggle with this all the time, and somehow my reading of material doesn’t answer it for me.

When I move objects I want to move them up/down and left-right but not in/out. This is irrespective of whichever viewport I happen to be looking into. Seems however that when I move a thing, or place its copy, it changes location in terms of depth too, usually to the construction plane.

I tried CPlane to redefine where the construction plane was for the operation, but that seems an unnecessary complication and didn’t work for me anyway.

How do you constrain movement so that the viewport’s Z doesn’t change? Thanks.

Usually but not always?

Are you using the Move command? Are you snapping to geometry that is not on the same viewport Z?