before不是过去式标志
例句
1、
So it's happened before， and it will happen in the future
这样的事情在以前发生过，在将来还会发生
2、
It's happened before， there were two government shutdown in the mid 1990s.
这在以前就曾经发生过，在1990年代中期有两个政府关闭。
3、
Interconnectivity allows for the possibility of a reading experience that was barely imaginable before.
互联性使我们有可能拥有在以前几乎是无法想象的阅读体验。
