The best way I've found to tell if something needs to be in a painting is to leave it out. If you don't miss it, you don't need it. A preliminary study that is entirely soft-edged, then, should reveal where a hard edge is needed. Still, if we want to avoid making too many hard edges, how do we know which ones are the most important?
I like to pretend that I'm on a ration of hard edges. If i am allowed only five, for example, I must choose very carefully where those five will do the most good. It turns out that most of us are pretty good at this kind of prioritizing. When it comes down to a firm decision, we can handle it quite well.
So...please make a soft-edged version of the image you brought home from class, or use this one:
Or this one:
Make a soft-edged version of the scene...all soft edges. If you see a hard edge, STOP PAINTING!
Re-wet the area, and keep going with soft edges. When you've presented the image without hard edges, you might have a basis for deciding where to use your skimpy ration to the greatest benefit of the painting. After you've exhausted your supply, grant yourself a couple more, if necessary, but draw the line at fifteen.