Using “it” as the subject of a sentence without the identity of “it” (the antecedent for “it”) being clear in a previous sentence (as in “It is often said that . . . .” or “It was decided that . . . .”) can be confusing.
When possible, replace “it” with an appropriate noun. Ask yourself: Who or what is “it”? Usually, you can determine the identity only from the sense of the whole passage.
Original: It is often said that editors are too picky.
Better: Many writers say that editors are too picky.
Starting a sentence with “This is” or “That is” is can be confusing. Does “this/that” refer to the preceding sentence’s subject, verb, the whole sentence, or the preceding four sentences? When possible and when needed, clarify the sentence.
Original: This is the crux of the problem.
Better: This ambiguity is the crux of the problem.