"One uses a knife when one eats meat."
Translation:Man bruker en kniv når man spiser kjøtt.
"en" means the number one. It is used for either "one thing" or "a thing" (en ting).
"man" means one as a general term for someone. An example would be "One does not simply walk into mordor." This use of "one" replaces a person doing something.
Hope that could help you grasp the differences. ^^;