Theoretical Aspects of MovementAs this would alter the lexical nature of the verb, we conclude that it would be
impossible. Yet if this position were simply vacant, like the subject position is at D-
structure, we might expect that it could be the landing site for some other moved
element. This, it turns out, is not true at all. Consider the following analysis which
indicates several movements step by step:
(95) a [DP e] Susan said [[DP who] helped [DP Fred]] D-structure
b who (did) Susan say [[DP e] helped [DP Fred]] after 1 movement
c who (did) Susan say [[DP Fred] helped [DP e]] after 2 movements
Both of these derived structures seem to be grammatical, but importantly they do not
mean the same thing. In (95b) Fred is interpreted as the one who is helped and the
interrogative pronoun who is the one doing the helping, as is indicated by the D-
structure in (95a). But in (95c), Fred is the one doing the helping and who is the one
helped. Under the assumption that -roles are assigned at D-structure, it cannot be the
case that (95c) was formed from the D-structure (95a), but must be related to another
D-structure, i.e. (96a):
(96) a [DP e] Susan said [[DP Fred] helped [DP who]]
b who (did) Susan say [[DP Fred] helped [DP e]]
The fact that (95c) cannot be interpreted in the same way as (95b) leads us to
conclude that the movement indicated in the former is impossible and that the object
cannot move into the vacated subject position.
The overall conclusion of this discussion then is that the empty positions that are
present at D-structure are of a different nature to the empty positions present at S-
structure which are created by movements: D-structure empty positions are vacant to
be moved into, S-structure empty positions are not. Obviously this demands an
explanation.
One possible account of the nature of empty extraction sites is that they cannot act
as landing sites for subsequent movements because they are occupied. As two
elements cannot occupy the same position and as we are not allowed to delete material,
this would block movement into this position. There are two problems that this
assumption faces: what element occupies the extraction site and why cannot we see it?
Given the above discussion, there is no choice as to the identity of the element that
occupies this position: it must be the moved element itself. No other element could
either be moved into this position or be inserted into it from elsewhere without
drastically changing the lexical information represented by the D-structure and this
would violate the Projection Principle. But then we seem to be forced to accept that
one element can occupy two positions.
We can get some understanding of this situation if we make the following
assumption: when an element moves, it leaves behind a copy of itself in the extraction
site. This copy is called a trace and is envisaged to be identical to the moved element
in terms of its grammatical and semantic properties. Thus the category of the trace, its
role in the thematic structure of the sentence and its referential properties are the same