"È semplice da dire ma non da spiegare."
Translation:It is easy to say but not to explain.
Well, in this case I think of "adjective + da + infinitive" as something is "adjective" when we come to doing that "infinitive" thing. And not using "da" here would be missing something and the sentence would seem to be incomplete. So you would say something like: È semplice dire questo, ma non semplice spiegarlo. I am not sure about the usage "di" here.
And for the case of "something + da + infintive", I would think of it like "something" is for the purpose of the "infinitive" thing. You would say: qualcosa da mangiare. But you wouldn't say ora da mangiare but instead ora di mangiare and that's because you cannot eat time :) this is from what I understand
Native speakers please correct me if I am wrong.
Thanks. yes, so 'da' in this case shows that 'it is easy (for the purpose of) to speak/say.
I dislike this sentence. Does it intend to mean in Italian that the act that you say but do not explain is easy, or that it is easy to say but it is not easy to explain?
I am in fact not sure which one the offered English translation would mean.
I feel the translation is correct, but in general it's a sentence that it doesn't make too much sense, whatever language you try to translate it into. At least, it's not easy to understand without a context.
As a context, you can think you are talking about the Pythagorean theorem (or another mathematical theorem); it could be easy to say what the theorem states, but it could not be easy to explain it.