before一般用于句末,后面没有任何表示【时间】的词语,用现在完成.强调【到现在为止】如：
I have never seen such an interesting film before.
如before后面用表示时间的词语或从句,用过去完成.如：
I had heard a lot about the city before I came here.
I had nver been to Shanghai before 2008.