We have a witness to the killing.

我们有一位这起杀人案的目击者。

I thought, at the time, that this was just a story but then I witnessed it with my own eyes many times.

当时，我以为那只不过是个传说而已，但是后来我亲眼目睹了许多次。

The year 1849 witnessed a great war in that country.

1849年那个国家发生了一场大战。