Can a gismu or lujvo have a "y" or "w" sound in it (by this I mean the English sounds)? Reading about the morphology again, it seems like this can't happen. Which means they only ever occur in attitudinals.
gismu or lujvo cannot contain the diphthongs uV and iV, where the English "y" and "w" sounds come from. These sounds occur in cmavo (the attitudinals), in zi'evla (aka fu'ivla, e.g. {kraui} which has two syllables {kra/ui}), and cmevla (e.g. ".uis.").