In "stick something on", is "on" an adverb or a preoposition?
to stick on is a phrasal verb, so it's "part of the verb". Context will define if it's serving as an adverb or preposition.
=> Stick it on. (adverb in this case)
=> Stick it on his head. (preposition in this case)
To stick on (to adhere)
To stick up (someone) (to rob)
To stick out (to be conspicuous)