How are actor nouns formed from verbs Are there general rules for forming nouns that refer to the one performing the action of a verb, such as cut -> cutter, run -> runner, "Jack the Ripper", etc.? I have been searching online but have only found guidance on forming gerunds, possibly because I don't know the grammar terminology to ask the right question.

There is no one-size-fits-all solution, and you have to be familiar with many patterns. To add to what Angelos have already mentioned, one important pattern is masu-stem as a noun.

* to peek / peeper
* to use magic / magic user → wizard
* to play sumō / sumō wrestler
* to raise sheep / sheep raiser → shepherd



"Jack the Ripper" is in Japanese, and is the masu-stem of the verb ("to rip/tear").