Here are some examples:
* Adjective: "She is a fast runner."
* Adverb: "The car moved fast."
* Verb: "He will fast for a week."
There are some rare instances where "fast" could be used as a noun, but it's not a common usage:
* A period of fasting: "He broke his fast with a light breakfast."
* A race: "The fast was a close one."
However, even in these cases, it's more common to use other words like "fasting period" or "race."