"It is hard to play the piano."
Thanks. However I didn't mean it quite like that. I meant using こと instead of の.
I'm pretty certain the sentence you wrote— [verb phrase] + の + こと —wouldn't be correct. I can't see what the sense of having both の and こと would be. Maybe you had "[noun] + の + こと" in mind when you wrote that?
Here is how I instinctively answered the question:
It marked my answer as being wrong, saying it wanted の instead of こと. However, it might well be that the only reason for this is because they may not have started adding any alternative Japanese answers for this sentence (since the course isn't officially released on web, and the apps don't have any Japanese typing...).
I was just trying to double-check that こと can be used, or to find out whether it might not be as natural as の in this sentence.
Sorry, I should have given a little more thought to my comment before I posted. I think I conflated noun omission (i.e.,「赤いの」 "the red one" in lieu of, for instance, 「赤いのシャツ」 "the red shirt") with the replacement of a generic noun (もの or こと) by の.
You're absolutely right that のこと, as I wrote in my original comment, would be redundant. I'm not sure that it is entirely ungrammatical but it's certainly awkward.
I completely agree with you that「ピアノを弾くことは難しいです。」is grammatically sound Japanese. As to whether こと is as natural as の, I'm not completely sure myself. My gut feeling is that の is more common in spoken (especially casual) language while こと may be more common in written language. Take the above with a grain of salt, however, as I'm still learning; I'd love for a native speaker to chime in.
EDIT: http://www.imabi.net/nominalization.htm is a good overview of this subject.
To chime in here: my textbook says the difference between の and こと is that の is used for things you have personal experience with, andこと is used for things that are general knowledge.
むずかしい - difficult (mentally) - used for tests, homework
たいへん - tough (physically) - used for labour, sports
Is ピアノはむずかしいです necessarily wrong? I thought I remembered a previous sentence that used ピアノ with either じょうず or へた and no verb.