Words that can be either prepositions or adverbs

   
about besides over
above between since
across beyond through
after by throughout
along down to
alongside in under
around like underneath
before near up
behind off within
below on without
beneath out
     
     
Words that are used only as prepositions   
against concerning onto  
amid despite toward  
among during towards  
at from unto  
bar into upon  
beside of with  
 
       
       
Words either prepositions or conjunctions   
except  
for  
till  
until