Can you please explain why do we say, for example, "What A good idea!" and, in this case, we omit the indefinite article A?
"A luck" doesn't make sense. "Luck" is an abstract entity like "happiness". For an instance of it we say "a bit of luck" or "a lucky thing". "That was a bit of luck" or "a lucky thing." Ideas are individual, and you can speak of "an idea."
Got it! Thanks!