"The shadow descends from the sky."
Translation:Umbra de caelo descendit.
To make it simple (it is of course more complex):
- a/ab = from (you leave a location)
- de = down from (you were in a high location and you go down)
But as I said, there are many other uses, e.g. de also means "about" (Librum de Romanis scribo : I write a book about the Romans) and a/ab is also used to express the agent (Liber a discipulis legitur : the book is read by the students).
Apparently, the increase of using prepositions in Latin goes hand in hand with passing from the antiquity towards the medieval period.