This question already has an answer here:
In QGIS 2.18 I draw polygons on a certain grassland-type on aereal imagery. Sometimes, there are other landcover-types within the patches of interest, so I'd like to draw polygons with holes.
In the example, I want to exclude the small forest-patches within the grassland. Is there any procedure other than creating a new layer for everything I want to exclude and then clip my grassland-layer?