1The exercise is so difficult that few of us can do it. 2 The exercise is too difficult for any of us to do.
Are they both correct? Yes, if you put a period at the end of the first sentence.
Could you please tell me why there is it in 1 but there isn’t it in 2 if they are both correct[FONT=宋]？[/FONT]Because the sentences/grammars are different.
The first sentence has two independent clauses separated by "that". So in the second clause, you have to refer back to the subject of the first clause "The exercise" with the pronoun "it". Of course, the second clause has its own subject "few of us".
The second sentence has only one clause and one subject, "The exercise", so we don't need to refer to the subject again with the pronoun "it".