Classical generative grammar (for example, Akmajian and Heny 1975) relates such sentence pairs by a rule of extraposition, where a clause is generated in subject position〖^4〗 and then moved, or extraposed, to the end of the sentence. The nonreferential subject it is then placed in subject position to satisfy the requirement for an overt subject in every English sentence. Note that when that-clauses are placed at the beginning of the sentence in this way, we cannot omit the complementizer that under any circumstances: