While numbers can function as different parts of speech depending on their role in a sentence, "eighteen" itself is not a traditional part of speech like a noun, verb, adjective, etc. It's simply a numerical value.
Here's how "eighteen" can act as different parts of speech:
* Adjective: "I have eighteen cats." (modifies "cats")
* Noun: "Eighteen is my lucky number." (acts as the subject)
* Pronoun: "I have eighteen, and you have twelve." (replaces a noun phrase)
* Numeral: "I have eighteen cats." (specifically, the number)