"It is exciting." Is context the only way to classify exciting in one of these grammatical terms (present participle, ing-adjectives, and gerund)?
In a comment, John Lawler wrote:
Like Schrödinger's cat, the -ing word in the example sentence is in an unknown state, since the sentence doesn't have sufficient context to distinguish between the possible categories it might occupy. The ambiguity might disappear in speech (it usually does), but the difficulty of analysis multiplies for those who feel lexical items must be labelled by category at each stage of a derivation; and most painfully for those who feel lexical items must keep the same label throughout a derivation. That just leads to a proliferation of labels, mostly epicyclic.