* When it's an adjective: "He's about to leave." Here, "about" describes the state of being ready or close to something.
* When it's an adverb: "The meeting went about as expected." Here, "about" modifies the verb "went" and conveys the manner in which the meeting went.
* When it's part of a phrasal verb: "He brought about the change." Here, "about" combines with the verb "brought" to form a single verb with a specific meaning.
In most cases, "about" functions as a preposition, indicating location, direction, or association:
* Location: "The book is about the history of art."
* Direction: "He walked about the city."
* Association: "They talked about their plans."
So, unless it's in one of those rare cases mentioned above, "about" is almost always a preposition.