What is the prefix for known?

There isn't a single prefix that directly translates to "known". However, depending on the context, you might use:

For "well-known":

* Pre-: like in "pre-existing" or "pre-eminent"

* Fore-: like in "foregone conclusion"

For "knowing":

* Cogni-: like in "cognition"

* Sci-: like in "science"

For "knowledge":

* Gno-: like in "agnostic"

* Know-: like in "know-how"

The best prefix to use will depend on the specific word you are trying to create and the nuance you want to convey.