Hi David, yeah MoI does not use shift or ctrl for that kind of a constraint - instead it is handled by Straight Snap being enabled or not in the bottom toolbar.

When it is enabled, you just move your mouse nearby to one of the axis directions and it will snap on to that direction when you get close enough. When it does snap you will see a marker point at the base of the line saying what axis direction is snapped on to (like x, y, z, or "normal" for a surface normal if you have placed the first point snapped on to a surface), and also an extended dotted line will show up also.

In general MoI is set up so that you don't have to be hitting modifier keys like shift or ctrl all the time.

- Michael