i can not go to bed,before finishing my homework 完全可以
after .before都可以作介词,也都可以作连词.意思一样,也就是用法不一样.作连词时,连接的两句话.作介词时,后面跟的是名词或动名词.
比如,after finishing my homework,i am going to bed.= i am going to bed after i finish my homework.不过从简洁的角度来讲,这句话用介词的用法更好些.