Il est très difficile de donner un nombre précis.
Why il est and not c'est? It's about a situation not a person. Très is an unmodified adverb so does that take priority over the adjective describing the situation?
Thanks for providing this fantastic link. Reminder for me: Basically you can say "C'est très difficile.", but "Il est très difficile de... .".
I looked through the link several times. I just couldn't find any reference to using de to distinguish between il est requirements and c'est requirements. It's important because de introduces a verb in this case so I don't understand what bearing it would have on the issue.
I think the general rule still stands. For more specific references, il est is used. More general phrases use c'est; More formal = il est; More informal/casual/common = c'est.
From this (still focussing on impersonal phrases only), C'est is used when the statement isn't followed by an adjective:
Whereas Il est can be followed by an adjective for impersonal phrases:
il est un bon livre
However, things change again if the impersonal phrase is followed by an adjective and a clause or infinitive:
c'est/il est vrai qu'il vit. or c'est/il est facile de manger
So back to this example, I agree with the view the the adverb here is not the crucial element. Rather, the phrase consists of all the elements in the last example above - it is impersonal; is followed by an adjective which is in turn followed by an infinitive (donner). This suggests to me that either il est or c'est can be used for this statement.
I found this link quite confusing. She says that you can dismiss adverbs and focus simply on the presence or absence of a modified noun.
Both about.com and authoritative commenters on Duolingo have said clearly that unmodified adverbs require il est while modified adverbs require c'est. They also indicate that adjectives referring to a person require il est while those referring to a situation need c'est.
Il est tard - It's late.....unmodified adverb
C'est trop tard - It's too late .......modified adverb
Il est fort - He is strong......adjective describing a person
C'est normal! - That's normal!.......adjective describing a situation
I get the sense that that she focuses on common spoken French but I was very surprised to read her view that adverbs don't count and can be simply dismissed from consideration when determining whether il est or c'est applies.