preposition

[proper rune]

Please be patient

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preposition

[shadow oriole]

"c'est difficile d'insister" uses "de" because "insister" is being refered to by "c' ". The verb is the subject of the sentence.
It's basically the same as "insister est difficile" but this latter construction is pretty rare

"c'est difficile à comprendre" uses "à" because you "c' " is referring to the thing that's hard to understand, not understanding itself.

it basically means the same as "c'est difficile de comprendre ça"
here, the action [comprendre ça] is the subject, so you can use "de"

[trail slate]

https://i.imgur.com/4BoY1yv.png

[shadow oriole]

thank you alba

[humble light]

In "It's difficult to insist", "it" doesn't refer to anything. The sentence could be rephrased as "Insisting is difficult"

In "It's difficult to understand", "it" does refer to something, the thing that is difficult to understand.