I’m trying to make a D, with one shape and another shape on top of it. The second shape isn’t becoming white space for some reason. Here’s the screenshot: i.imgur.com/jf8wmOl.png
You need to change the path direction of the inner path.
Or just use Layers > Correct Path Direction (Cmd-Shift-R) to let Glyphs guess all path directions automatically. Works 99% of the time.