WordNet sense 1 (being the only one; single and isolated from others):

lone, lonesome, sole, solitary

WordNet sense 2 (existing alone or consisting of one entity or part or aspect or individual):

single

WordNet sense 3 (exclusive of anyone or anything else):

alone

WordNet sense 4 (excluding much or all; especially all but a particular group or minority):

exclusive

WordNet sense 5 (never except when):

only if, only when

WordNet sense 6 (without any others being included or involved):

entirely, exclusively, solely

WordNet sense 7 (and nothing more):

but, just, merely, simply