Convert to lines (keyboard shortcut ⌥⌘L)
Select any object – Oval, Rectangle, Rounded Rectangle, Arc, Polygon or Spline – and hit ⌥⌘L. This will convert the object into lines, which you can work on, manipulate etc., as you would any other line.
Ovals, arcs, and splines convert to polylines (a large number of very short lines joined end to end – in fact simply a polygon with a very large number of sides).
Rectangles and polygons convert to their constituent lines. Rounded Rectangles convert to four lines making up the sides and polylines making up the radiused corners.
Tip: When working with imported objects via DXF, sometimes what appears to be a line can in fact not be a line but rather a "single-sided polygon" (a contradiction in terms of course!). You find that you can't manipulate it like you would a normal line – you can't cut it and so on. Try "Convert to lines" – this often fixes the problem. Your "line" may turn out to be lots of short lines joined end to end in a straight line; or it may often turn out to be simply a single line, encoded as a polygon.Convert to polygon (keyboard shortcut ⌥⌘P)
This pretty much does the reverse of "Convert to lines". It will convert any set of lines that are joined end-point to end-point into a polygon, the sides of which will be the original individual lines.
Tip: If you have previously used "Convert to lines" on an object, this won't convert it back to what it was. A circle, for example, converted to lines and then converted to a polygon, won't be a circle again – it will me a many-sided polygon that pretty much looks like a circle. And a rectangle will be a four-sided polygon – which is a subtly different beast: you can drag any of the corners of a four-sided polygon, whereas you can't with a rectangle.
Tip: The Action menu can also be accessed directly on the screen: select an object or objects and right-click or ⌃-left-click (control-left-click).