5.2.3.3 Unaccusatives and ergatives
This is a case of raising, as introduced in chapter 3. In (62a) the subject of the lower clause is raised into the subject position of the raising verb seem, demonstrating that this position must have been empty at D-structure. In (62b), however, the thematic subject of the lower clause does not move out of this clause. In this case the subject position is filled by another expletive element it. It would be ungrammatical for this position to be left empty, an indication that all English sentences must have subjects regardless of whether one is semantically demanded or not. We will return to this observation in the next chapter. Note however that this expletive subject differs from the one used in there constructions, though their function (to fill a vacant subject position) seems to be similar. It would be ungrammatical to use a there in raising structures and it in there constructions: