Many verbs can be suffixed by er, which makes them nouns. However, some nouns made this way refer to persons, while others refer to tools or actions. For example:
- pay: payer (the person who pays)
- cook: cooker (the tool used for cooking)
- pray: prayer (the action of praying)
I haven't seen people calling a payment tool (such as Square Reader) payer. Similarly, I haven't seen people calling someone who cooks cooker. But people do call someone who teaches teacher.
So how can language learners tell, for an unknown verb, which concept it means when suffixed by er? Furthermore, by the pigeonhole principle, you can't precisely fit three concepts into one form. Are there any rules or heuristics governing er suffix?