I can only find the expression "in decline" in dictionaries. Can "decline" be used in the way of "on the decline", as it is with "increase" or "decrease" ("on the increase/decrease")?
***NOT A TEACHER***Great question! Yes, both "in decline" and "on the decline" are used quite regularly by native speakers. (1) I wish I could tell you the difference -- but I can't. (2) They seem to be interchangeable quite often. (3) I get the "feeling" that "in decline" is often (but not always!!!) rather negative: Good manners seem to be in decline throughout the world (No further comment about that subject!!!). (4) My feeling (which could be 100% wrong) is that "on the decline" is usually (not always!!!) rather positive: Illiteracy in country X is definitely on the decline. In other words, maybe (maybe!!!) "in decline" refers to something that is hard to measure and may be controversial: Some people say nation X is in decline; "on the decline" seems to be something that can be measured better: H1N1 seems to be on the decline in the United States.