It's a determiner and can also act as a pronoun.
Here's how it works:
* Determiner: "Either" is used to refer to one or the other of two things, often in a choice situation.
* Example: "You can have either the apple or the orange."
* Pronoun: "Either" can also stand in for a noun phrase.
* Example: "I'm going to the store, but I don't know which one; either will do."
Prepositions, on the other hand, show the relationship between a noun or pronoun and another word in the sentence. They often indicate location, time, or direction.
Examples of prepositions:
* in, on, at, to, from, by, with, for, etc.
Let me know if you'd like more examples or explanation!