Image courtesy of Bifrost and Beyond
While their names are readily applied linguistically to define them (Urd=past, Verdandi=present, Skuld=future), their story in the Norse Mythology makes them much more interesting and complex.
Urd represents the past, but that past is not just our life. It means everything that happened leading up to the present state of our life. In other words, our life is shaped by our choices and the influence others have had on it through time. It also includes our ancestry and the ways in which our heritage or cultural background place us in the world. A huge infrastructure, of good and bad, worked to make us who we are.
Of course, Urd, representing all of that past experience, brings us to Verdandi, the present or, more specifically, what is happening now. Just as everything we've done to this moment has made us who we are now, in the current moment, we make choices that affect who we are and which direction we will go as the future arrives.
A very interesting thing occurs at this point in Völuspà , verse 20, where the Norns are introduced. The line immediately following "the second Verdandi" does not introduce Skuld, rather refers to "they" carve on sticks or cut Runes - Skáru á skiði. To me, this implies that Urd and Verdanadi cut Runes, which result in Skuld.
The message that I take from this is that our past (all of the parts of the past mentioned above) made us who we are today and that, in turn, will guide us into the future. Still, Skuld isn't exactly or simply future, rather relates to should or shall be. Of course, the only things we can know for certain are those things that have happened already and which are happening now. The future is a mystery. So, mysterious Skuld may represent what "should be", but as we move into the future, we still have the option to change that path or direction through our choices. Although the Norns hold our ultimate destiny in their hands, how we arrive there is up to us through our choices and the experiences we make for ourselves.
Going back to "skáru á skiði", think of it like this. We know our Urd; it has been carved. We live in our Verdandi; it is being carved. Although our Skuld is a mystery, our past and present will carve it.