Now that you know how to do one line, doing the other three directly follows what you've already done. Just define the moving points C and D in terms of the static points C ₀ , C ₁ , D ₀ , and D ₁ , then define the lines between B and D, D and C, and C and A using the same parametric definition of the line as for the line between A and B. Click the arrow next to "the moving lines" to see how those equations can look.

You can also click the arrow next to "the scaffolding" and turn on the dotted lines and see the straight paths followed by each of the corners.