We can use before to mean 'at any time before now'. We use it with a present perfect tense (have + past participle).
Have you seen this film before? I ve never been here before
Before can also mean 'before then', 'before the past time that we are talking about'. We use a past perfect tense (had + past participle).
She realized that she had seen him before.
In expressions like three days before, a year before, a long time before, the meaning is 'before then'. We use a past perfect tense. (See 20.4 for an explanation of the difference between before and ago in these expressions.)
When I went back to the school that I had left eight years before, everything was different.
Fact
Detroit policeman William L. Potts is credited with inventing the modern street traffic light in 1920. He worked out an electric light system that allowed him to control three street intersections from one tower He picked the red, yellow and green because railroads used them. .. More >>