before单独使用有两者情况：1.放在句尾.句子是否定句.这时该句常常用现在完成时.如：
I have never seen him before.
2.before 后不跟句子,那before 就是个介词,而不是连词了.其后就要用动名词.句子时态看具体情况而定.
He said nothing before leaving.