* Noun: This is the most common usage, referring to a person who runs, such as a marathon runner.
* Adjective: It can describe something that runs, like a "runner" bean or a "runner" rug.
* Verb: It can be used as a verb in the present participle form, like "The engine is running."
To determine the correct part of speech, you need to look at the sentence and how "runner" is used.