You probably need to go to Preferences >> Constraints angles ... and set the constraint angles that you wish to use either generally or for a particular drawing.
The shift key will then constrain an object you are drawing, or moving, to the various angles you set there – see also http://www.whisstock.com/realcadd/page_ ... ub_page=03
AS Eric says, you do have to hit the shift key after you select the object etc. And be careful not to grab it by a handle (the little black squares at ends, corners etc) else you will change its shape, length etc, rather than move it.
With an existing line, drawn to a constrained angle or not, you can extend or shorten it, maintaining the exact drawn angle, by holding down the Option key, after you have selected it (by a handle) but before you begin to drag. Extending or shortening holding down the Shift key will change the line to one of the set constrained angles.
See also http://www.whisstock.com/realcadd/page_ ... tool_id=03
- the section on "Dragging a line with its handles"
You can also use Snap to move or extend objects to line up with, or snap to, endpoints, centers etc of other objects.
Cheers -- George
PS - after reading you post I realized that I had not mentioned in the manual that constraints worked for both drawing a line (or polygon) and moving any object – I have now added that information to the Preferences >> Constraints section.