I would like you to tell me if this sentence is correct: It is well known to be very difficult, or whether this one would be better: It is well known that it is very difficult.
Thanks in advance
They are both correct sentences. The it in the first sentence, as well as the second it in the second sentence, refers to something already present in the context. The it at the beginning of the second sentence, however, does not. "It is well known that it is very difficult" means "(The fact) that it is very difficult is well known."