Here's an example:
* The cat sat near the fireplace.
In this sentence, "near" introduces the prepositional phrase "near the fireplace" and tells us the location of the cat.
However, "near" can also be an adjective:
* The near future.
In this case, "near" modifies the noun "future," indicating that the future is close in time.
So, whether "near" is a preposition or an adjective depends on its function in the sentence.