Suggested answer for Exercise 4
The sentence Mary was sent a message is a passive sentence with the so called double object construction. The main verb is in V in the D-structure. The theme argument a message occupies the specifier of the VP, the agent DP is absent in passive structures, the vP hosting the agentive DP is replaced by the passive vP. The third argument DP is in the complement position of the main verb and it is assigned a goal thematic role by the verb. The DP a message receives accusative Case in its base position, so it does not have to move. The DP Mary, however, needs to move to a Case position. First it moves to the specifier of the lower vP, which is the position for DPs with a goal thematic role in double-object constructions. In an active sentence, the DP Mary can stop in this position, since it can receive Case here as it can be attested in the sentence in (11).
|(11)||John sent Mary a message.|
In our sentence, this position cannot be the final position of the DP Mary the v head of the middle vP is occupied by the passive morpheme en, which does not assign accusative Case. So the DP has to move on to the next position where it has a chance to receive Case. This position is the specifier of IP, where the finite I head assigns nominative Case to it. The main verb has to adjoin to the lower v, which is an abstract bound morpheme. This complex head has to move on to the middle v head containing the passive morpheme, which is also a bound morpheme. The upper vP is the tense vP where be is inserted as a dummy auxiliary as the thematic verb cannot move further on from the passive vP. Be together with the tense morpheme moves on to I containing the zero agreement morpheme.